Abstract:
A system enabled for fault-tolerant symmetric multi-computing using a group of nodes is described hereon. A symmetrical group of nodes networked using a reliable, ordered, and atomic group-to-group TCP communication system is used in providing fault-tolerance and single system image to client applications. The communication between the client and the group is standards based. The processing load is shared among a group of nodes with transparent distribution of tasks to application segments. The system is fault-tolerant in that if a node fails remaining replicas if any continue service without disruption of service or connection. Nodes may be added to or retired from the group in a manner transparent to the client as well as server applications.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/505,091, filed Sep. 22, 2003, which is incorporated by reference in its entirety. 

   This application is related to the co-pending U.S. application Ser. No. 10/942,763, entitled “Group-to-Group Communication Over a Single Connection,” (Inventor: Anilkumar Dominic), filed Sep. 15, 2004, which is herein incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fault tolerant multi computing system using group-to-group communication scheme. 
   2. Brief Description of the Related Arts 
   For the optimal resource utilization, flexibility and reduced management costs the industry demands solutions based on a “utility computing” model where processing power and storage capacity can be added as need and resources are provisioned dynamically to meet changing needs. Conventional mainframe solutions are beyond the reach of average enterprises due to high cost. There are large number of high performance but low-cost “blade servers” and networking technologies available in the market. However, a solution that aggregates these resources efficiently and flexibly and can run wide range of applications to meet the utility computing needs does not exist today. 
   The client-server paradigm is popular in the industry due to its simplicity in which a client makes a request and server responds with an answer. To enable this paradigm, a popular communications protocol used between a client and a server in a communication network is, transmission control protocol/Internet Protocol, or simply, “TCP/IP.” In the communication network, a client (or client system or machine) views a server (or server system or machine) as a single logical host or entity. A single physical server is often incapable of effectively servicing large number of clients. Further, a failed server leaves clients inoperable. 
   To address the shortcomings of a single physical server, cluster configurations having many servers running in parallel or grid to serve clients were developed using load-balancers. These configurations provide potential benefits, such as, fault-tolerance, lower cost, efficiency and flexibility comparable to mainframes. However, these and other benefits remain largely unrealized due to their inherent limitations and lack of a standard platform most applications can build on. 
   In addition to physical clustering, conventional software systems have also made efforts to introduce clustering at application level and operating system levels. However, shortcomings of such software configurations include instances where clustering is embedded in the application results in limited usage of those applications. Similarly, although operating system level clustering is attractive, conventional efforts in these areas have not been successful due to large number of abstractions that must be virtualized. 
   In contrast to physical server and software application and operating system clustering, network level clustering does not suffer from either of the problems and provides some attractive benefits. For example, the ability to address the cluster of server nodes as a single virtual entity is a requirement to be useful in client server programming. Further, the ability to easily create virtual clusters with a pool of nodes adds to better utilization and mainframe class flexibility. 
   A conventional network level-clustering platform must be generic and usable by a wide range of applications. These applications range from, web-servers, storage servers, database servers, scientific and application grid computing. These conventional network level clusters must enable aggregation of compute power and capacity of nodes, such that applications scale seamlessly. Existing applications must be able to be run with minimal no or changes. However, conventional network level clusters have had only limited success. 
   To the extent there has been any success of the Symmetric Multi-Processor (SMP) architecture, it can be attributed to the simplicity of the bus, which made processor and memory location transparent to applications. For clustering too, simplicity of a virtual bus connecting server nodes provides node location transparency and node identity transparency. However, such conventional systems lack the capability of allowing a bus to be directly tapped by client applications for efficiency. Similarly, buses based on User Datagram Protocol (“UDP”) packet broadcast and multicast lack data delivery guarantees, resulting in application level clustering. 
   The single most used protocol with delivery guarantees by the industry is TCP/IP. The TCP&#39;s data delivery guarantee, ordered delivery guarantee and ubiquity, makes it particularly desirable for virtualization. However, TCP&#39;s support for just two-end points per connection has limited its potential. Asymmetrical organization of processing elements/nodes that have pre-assigned tasks such as distributing incoming requests to cluster are inherently inflexible and difficult to manage and balance load. Asymmetrical nodes are often single point of failures and bottlenecks. In order for MC (Multi Computing) to succeed, there is a need for symmetrical organization as opposed asymmetrical node organization. 
   Another problem with asymmetry in a client-server environment is latency. Switches and routers employ specialized hardware to reduce latency of data passing through. When data must pass through node&#39;s UDP/TCP/IP stack, it adds significant latency due to copying and processing. Hence, in order to achieve optimal performance, systems must avoid passing of data through intervening nodes having asymmetric organization. However, if a server node&#39;s CPUs must handle large amount of network traffic, application throughput and processing suffers. Thus, conventional systems must use hardware accelerators such as specialized adaptor cards or Integrated Circuit chips to reduce latency at the endpoints and improve application performance. This increases system costs and complexity. 
   Low-cost fault-tolerance is a is highly desired by many enterprise applications. Solutions where fixed number of redundant hardware components are used suffer from lack of flexibility, lack of ability to repair easily and higher cost due to complexity. Solutions today offer high availability by quickly switching services to a stand-by server after fault occurred. As the stand-by systems are passive its resources only not utilized resulting in higher cost. In the simplest yet powerful form of fault tolerance by replication, the service over a connection continue without disruption upon failure of nodes. 
   On traditional clusters, an active node performs tasks and passive nodes later update with changes. In many instances, there are fewer updates compared to other tasks such as query. Machines are best utilized when load is shared among all replicas while updates are reflected on replicas. Replica updates must be synchronous and must be made in the same order for consistency. With atomic delivery, data is guaranteed delivered to all target endpoints, before client is sent with a TCP ACK indicating the data receipt. In the event of a replica failure, remainder of the replicas can continue service avoiding connection disruption to effect fault-tolerance. Non atomic replication lacks usability. Specifically, when a client request is received by replicas of a services, each produce a response. As client views server as a single entity it must be made sure that only one instance of the response is sent back to client. Similarly, when multiple client replicas attempt to send same request, it must be made sure that only one instance is sent out to server. Conventional systems often fail to provide atomicity, and therefore, lack fault tolerance avoiding connection disruption. 
   Another problem with conventional clustering systems is load balancing. As with any system, the ability balance load evenly among nodes is necessary for optimal application performance. However, conventional clustering systems provide only limited support for standard load balancing schemes, for example, round-robin, content hashed, and weighted priority. Moreover, many conventional clustering systems are unable to support implementing application specific load-balancing schemes. 
   Many services have load levels varying significantly in a cluster depending on time. Running processes may need to be migrated for retiring an active server. Conventional cluster systems often lack support for adding or removing nodes/replicas to cluster in a manner that is easily performed and without disrupting the service. 
   A number of attempts have been made to address network level virtualization. However, each attempt has still resulted in significant shortcomings. For example, one conventional solution is a device for balancing load in a cluster of Web-Servers is popular in the industry. This load-balancing device, which is also disclosed in U.S. Pat. Nos. 6,006,264 and 6,449,647, switches incoming client TCP connections to a server in a pool of servers. A conventional server for this process is Microsoft&#39;s Network Load balancer software, which broadcasts or multicasts client packets to all nodes by a switch or router. However, once a connection is mapped, the same server handles all client requests for the life of TCP connection in a conventional one-to-one relationship. 
   A problem with conventional systems such as the ones above is when a service is comprised of different types of tasks running on nodes, it fails to provide a complete solution because any mapped server that would not run all services client would request over a connection results in service failure. This limits the use of such systems to web-page serving in which only one task of serving pages is replicated to many nodes. In addition, any mapping of devices implemented external to a server is a bottleneck and results in a single point of failure. Further, because a connection has only two end points, replication is not supported. Therefore, with such single ended TCP, updates are not reflected on replicas, and hence, there are considerable limits on usability. 
   To address some of the shortcomings of the above conventional systems, other conventional systems attempted to distribute client requests over a connection to nodes serving different tasks. Ravi Kokku et al disclosed one such system, in “Half Pipe Anchoring.” Half pipe anchoring was based on backend forwarding. In this scheme when a client request arrives in the cluster of servers, a designated server accept the requests and after examination of the data, forwards to an optimal server. The optimal server, given with connection state information later responds to the client directly after altering the addresses to mach the original target address. Here a single TCP end-point is dynamically mapped to nodes to distribute requests. This scheme is an example of “asymmetric” approach in that an intervening node intercepts the data and distribute it based on data content. 
   Another conventional system attempting to achieve asymmetric organization is disclosed in two whitepapers by EMIC Networks Inc. In this conventional system, a designated node intercepts and captures incoming data and later reliably delivers it to multiple nodes, using proprietary protocols. Sometimes only one node is permitted to transmit data, and data must be transmitted first to a designated server which later retransmits it to client. Here also the single end-point is dynamically mapped and the TCP connection terminates at the intervening node where replication is initiated. This scheme is another example of “asymmetric” approach in that an intervening node intercepts the data and replicate it. 
   Both schemes described above maintain the TCP definition of two endpoints, although they may be mapped to different nodes. Replication in these conventional schemes is performed at the application level using proprietary protocols. Further, these conventional schemes employ asymmetric node organization, where select nodes act as application level router that distributes requests. However, such asymmetry results in scalability limitations as noted in “Scalable Content Aware Request Distribution in Cluster Based Network Servers” by Aaron et al. These limitations include a single point of failure, data throughput bottlenecks, suboptimal performance due to higher latency, and lack of location transparency. 
   Therefore, there is a need for a symmetric system and a method for using the current definition of TCP&#39;s two endpoints to provide m-to-n connections (m, n, being any integer, which may be the same to different). 
   SUMMARY OF THE INVENTION 
   The above mentioned and other requirements are met by a group of nodes networked symmetrically using an atomic group-to-group TCP communication system. The system delivers a single system image to the client as well as server applications. The communication between the client and the group is standards based in that any standard TCP/IP endpoint is able to seamlessly communicate with the group. 
   The system is fault-tolerant in that if nodes running an application fails, a set of remaining application replicas in the group continue service without disruption of the connections and service. Nodes may be added to or retired from the group dynamically, to maintain a certain quality of service in a manner transparent to the applications. For the purpose of balancing load among nodes or retiring a node, system transparently migrates active services and re-distribute tasks within the group. 
   Applications running on nodes of a group are able to view and operate remainder of the group as single virtual entity simplifying client/server application programming and resource management. An embodiment of present invention allows for dividing applications into one or more segments independently running over group nodes, often in a manner transparent to the applications that no code change is required. 
   The system shares the processing load among a group of nodes by dynamically and transparently distributing incoming tasks over a connection to various application segments. A single request arriving over a connection may be serviced by multiple segments working in cohesion, enabling finer distribution of computation or processing among the nodes. The system allows for multiple instances of a segment run in parallel. Requests are delivered to the instances selected based on schemes such as round-robin, least loaded node, affinity based, content hashing. 
   Incoming requests over a connection are delivered atomically to multiple segment instances for fault-tolerance. The results are optionally compared and a single instance is output. Upon failure of segments/nodes, remaining segment instances continue service without disruption of the connection. 
   The system allows for flexible and external management of the system, by distributing tasks in a fine-grained fashion controlling and configuring filters at the connection endpoints. When retired, load responsibilities of the node are migrated to another node selected using schemes such as lowest loaded, round robin or an application specific scheme. The system automatically and dynamically adds resources to the group from a pool to meet changing needs. Similarly, nodes are retired and provisioned dynamically and automatically. The system maintains specific quality of service adding or retiring resources automatically and dynamically. 
   In addition, it is noted that the system maintains specific quality of service adding or retiring resources dynamically and automatically. For balancing load among nodes or retiring a node, system transparently migrates active services to other nodes. Distribution of tasks is controlled external to applications by management software. Client applications view and operate a cluster of nodes as a single virtual entity. A node communicates with other nodes of a group by creating a connection to the virtual entity represented by the group. Application running on a node is also able to view and operate remainder of the group as single virtual entity simplifying the programming and management. 
   The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention has other advantages and features which will be more readily apparent from the following detailed description of the present invention and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
     Figure (“FIG. ”)  1   a  is a generalized diagram of a symmetric multi-computer system with fault tolerance, load distribution, load sharing and single system image in accordance with one embodiment of the present invention. 
       FIG. 2   a  is a generalized diagram of communication system constructed in accordance with one embodiment the present invention. 
       FIG. 2   b  is a block diagram illustrating a communication system in accordance with one embodiment of the present invention. 
       FIG. 2   c  illustrates a block diagram of organization of higher-level components for implementation of a communication system in accordance with one embodiment of the present invention. 
       FIG. 3   a  illustrates a block diagram of a computing system for a communication system in accordance with one embodiment of the present invention. 
       FIG. 3   b  illustrates a block diagram of a computing system for a communication system having providing offloading of a main processor in accordance with one embodiment of the present invention. 
       FIG. 3   c  illustrates a block diagram of a computing system for a communication system providing offloading of a main processor to dedicated hardware/accelerator chips in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention includes a fault-tolerant multi computing system based on symmetric group-to-group TCP communication system, that efficiently shares load among a group of nodes by distributing tasks in a manner transparent to client and server applications. 
   Introduction 
   It is noted that for purposes of ease of understanding, a node refers to a connection on a network. For example, a data processing device such as general purpose computers, other devices having a microprocessor, or software configured for operation in a device may be referenced to as a node. A group refers to a collection of one or more nodes organized symmetrically. An application segment refers to a segment of an application that may serve in conjunction with other application segments running on various group nodes. An application is comprised of one or more application segments and an application segment is comprised of one or more processes. 
   A sendHead refers to a transmitting end of a TCP connection, which controls data transmission and maintains the transmission state at the node. A receiveHead refers to the receiving end of a TCP connection, which controls data reception on connection and maintains data reception state at the node. An active sendHead refers to the sendHead that is designated to have latest transmission state information, for example, sequence number of data and sequence number of last acknowledgement. 
   A bus controller refers to a node that controls and/or coordinates connection establishment and termination process with a peer group. A signal refers to a message exchanged within a node group over a logical bus. When a source and a target of a signal are within a same node, no signal is sent out, although it may amount to the effect of receiving it internally. A TCP end-point refers to an end application entry point such as socket at which data and acknowledgment for received data originates. A request refers to a select segment of incoming data stream, for example, a client request for service. 
   Load Sharing and Load Balancing 
   Referring now to  FIG. 1   a , illustrated is a symmetric multi-computer system in accordance with one embodiment of the present invention. The server group ( 912 ) is comprised of a number of nodes  900   a,b,c,d,e,f . The input stream ( 910 ) of TCP connection ( 909 ) has multiple endpoints  910   a,b,c,d,e,f  that span over the group nodes. Similarly the output stream ( 911 ) of the same connection is comprised of endpoints  911   a,b,c,d,e,f.    
   The application is comprised of three segments ( 913 ,  914 ,  915 ) running over the entire group with two instances for each application segment  913   a,b ,  914   a,b ,  915   a,b . By programming the communication system, the segments are delivered with specific tasks based on criteria such as operations they perform, the data they manage. By configuring the data delivery such a way that specific subsets of requests for services are delivered to specific instances of applications, segmentation of application is achieved, in many cases without code change to existing applications. Applications may be segmented in many ways, examples include segmenting based on type or kind of requests a segment can handle, a hashing algorithm based on data content or connection information such as sequence number etc. It is also trivially possible that application is divided in to segments by programming them into different segments. 
   The group nodes are paired as replicas  900   a,b ,  900   c,d  and  900   e,f  such that each pair run two instances of the application segment  913 ,  914 ,  915  respectively. Upon failure of a segment say  900   a  the pair  900   b  continue service without disruption. If failure of an instance say  900   a  happen while transmitting, the other instance  900   b  will send the remainder of the response to peer avoiding disruption of service. Similarly a new application segment instance may be added to a group so as to increase the fault-tolerance due added instance available to continue service in the face of failures. This may be done for example by creating a new process running application segment instance and then getting it added to group so that requests are distributed to it accordingly. 
   In one mode of operation, non-empty subsets of groups are delivered with requests in specific orders such as round-robin and weighted priority that requests are essentially distributed among said non-empty subsets so as to balance the load on nodes. 
   In one mode of operation one or more replicas are delivered with a task, and after the task is complete the results from instances are sent out through the connection without regard for others. In another mode of replica operation, one or more replicas may be delivered with same task. The relevant replicas then execute the operation in parallel and produces results. An output filter installed at the output stream of the group-to-group communication system compares results and a single instance of the result is sent out to a peer group whereby the group appear as a single entity to peer group. The selection of output instance transmitted to peer group depends on the policy set in the filter such as equal outputs, majority agreement, correct result or successful operation output etc. Selection of the policy depends on the application. Upon failure of a transmitting segment instance, a replica takes over and continues transmissions without connection disruption. 
   When output comparison result by the output content filter indicates differing outputs produced by nodes, a subset replicas are considered faulty and excluded from further service over connection while remaining endpoints continue service without connection disruption. In an embodiment having the exclusion of an endpoint, such exclusion is based on schemes where majority of endpoints agree on a result to exclude others. Alternatively, exclusion of endpoints may occur where an operation failed. Exclusion of an endpoint may also be from any application specific scheme that is programmable with filter. 
   In yet another mode of operation, the replicas are delivered with operations that result in state changes such as modified data in memory and storage. This way replicas maintain a consistent state. When operations that does not affect consistency between replicas such as read operation, the task is delivered to only an instance of the replica. This enable balancing of load between the replicas. 
   Node Addition and Retirement 
   The filters at the connection end point of the TCP group-to-group communication system, enable fine-grain control of data delivery to application segments. By dynamically configuring filters certain tasks are delivered to certain nodes, enabling external control over the delivery of task requests to node. Thus flow of requests to application segments are controlled like a switch for fine task distribution among nodes. 
   The group may be added with nodes any time. A newly added node may share load from existing connections and new connections. For existing connections, nodes join the service and starts accepting tasks arriving on it. When necessary load among nodes are balanced by migration of tasks. 
   For node retirement, load responsibilities of the node are migrated to another, selected using schemes such as lowest loaded, round robin or an application specific scheme. While retiring, waiting for smaller tasks to finish while not accepting new tasks, the nodes are freed-up completely. When long running tasks are involved, the migration of tasks such as system level process migration is used. With process migration the entire context of application process such as stack, data open files are moved to another node transparently. Nodes communicate with other nodes of a group creating a connection to the address of the virtual entity represented by the group. This provides all the above features for communication between group nodes. 
   Automatic Provisioning 
   The system automatically and dynamically adds resources to the group from a pool to meet changing needs. Similarly, nodes are retired and provisioned dynamically and automatically. The system monitors the quality of the service delivered to the clients and maintains specific quality of service adding or retiring resources. The operations can be done external to the system and are potentially transparent to the peer group. 
   Underlying Group-to-group Communication System 
   Referring now to  FIG. 2   a , illustrated is a communication system used in one embodiment of the present invention. The communication system includes a TCP connection  130  that couples between a first group  120  and a second group  160 . By way of example, the first group  120  has a first, second, and third member nodes  100   a ,  100   b ,  100   c  and the second group  160  has a first and second member nodes  150   x  and  150   y . The member nodes in either group are organized symmetrically in that each node has equal access to a TCP connection and operates independently and in parallel. A first data stream  110  and a second data stream  111  can flow between the first group  120  and the second group  160  of the communication system. 
   A first application segment  135  and a second application segment  136  constitute a server application on  120 . The first application segment  135  has a set of replicas  135   x ,  135   y  and the second application segment  136  also has a set of replicas  136   x ,  136   y . The application segment replicas  135   x  and  135   y  runs over nodes  100   a  and  100   b  respectively while the replicas  136   y  and  136   x  runs over nodes  100   b ,  100   c  respectively. A client application at group  160  is comprised of an application segment  151  with replicas  111   a  and  151   b.    
   Application segments  135  and  136  of the first group  120 , communicate over the connection  130  with segment  151  of the second group  160 . The two data streams  110  and  111  of the connection  130  follow TCP protocols. The connection  130  may have three different connection end points  130   a ,  130   b ,  130   c  at the first group  120  and two different connections end points  130   x  and  130   y  at the group  160  on the same connection. 
   Each group  120 ,  160  is assigned a respective group Internet Protocol (“IP”) address  121 ,  161 . Groups view each other as a single entity while being composed of nodes. Communications between two groups  120 ,  160  are addressed to each other through the group IP addresses  121 ,  161 . When a request from say segment  151  arrives at the first group  120 , it is viewed as data coming from group IP address  161 . Similarly, the second group  160  sends data targeted to group address  121 . 
   The endpoints  130   a ,  130   b  and  130   c  at the first group  120  may be set such that one or more of the application segment replicas  135   a ,  135   b ,  136   a ,  135   b  are delivered with an incoming request. Examples of the different policies by which data delivered to application segments are, all replicas, one replica, all application segments and select application segments, target determined based on request content, based on round-robin request distribution, based on a hashing scheme to map request to a specific node and weighted priority etc. 
   Either of the endpoints  130   x  or  130   y  at the second group  160  may send request to server group  120 . One or more of the receiveHeads at the endpoints  130   a ,  130   b ,  130   c  at the first group  120  receives the data depending on the settings. The endpoints  130   a ,  130   b ,  130   c  at the first group  120  may send response data which is received at the endpoints  130   x ,  130   y  at the second group  160 . Application processes wanting to receive certain or all in coming data are guaranteed to have received it before acknowledging client with the receipt of data. In order to maintain TCP&#39;s sequential order of data transmission, the TCP sequence numbers are assigned in sequential order before data transmission starts. 
   Optionally, duplicate data output by replicas  151   a  and  151   b  in the second group  160  are reduced to a single instance to be transmitted to the first group  120  by the communication system. Similarly, optionally output of replicas of application segments  135 ,  136  in the first group  120  may also be reduced to one. It is not necessary that replicas of  135   a ,  135   b ,  136   a ,  136   b  must produce outputs since in many cases request is delivered to only one replica depending on the settings. 
   The communication system in accordance with the present invention provides client/server requests and responses that are beneficially atomic. That is, they are sent or received as a contiguous sequence of bytes, enabling multiple processes over two groups send and receive data over a single connection. 
   The protocol between groups  120  and  160  is TCP and data is guaranteed to be delivered in the sequential order it was sent as per conventional TCP. When targeted to multiple end points, it is guaranteed that data is delivered to all targets end points or before client is sent with TCP ACK segment indicating the receipt of data. Optionally, when replica outputs must be reduced to transmission of a single copy output, it is guaranteed that output is atomic in that data is transmitted if all nodes output same data. However when results don&#39;t match applications may optionally choose output to transmit based majority agreement or correct or successful result, etc. 
   With application segmentation, application processes are typically delivered with only select portions of an incoming data stream for processing. For example, requests arriving on the second data stream  111  may be delivered to select applications segments. The order of the delivery of data to application processes must be guaranteed to be the order in which it was sent as specified by RFC 793. I.e. before certain data is delivered to an application segment all preceding data arrived in the stream must be successfully delivered to its target application end-points. 
   Referring to  FIG. 2   b , the first group  120  is comprised of the first, second, and third nodes  100   a ,  100   b ,  100   c . The connection  130  between the first group  120  and the second group  160  has the outgoing and the incoming data streams  110 ,  111 . Each node  100  has a group-to-group communication stack  130   a - c  respectively. The delivery of data to all of the nodes is through a switch  141   a - c  coupled with the respective nodes  100   a - c . No assumption about the delivery guarantees to switch  141   a - c  by the underlying hardware is made, since popular hardware technologies such as Ethernet are unreliable. Delivery of data to each node  100   a - c  or any of its subsets may be selective or no delivery at all is possible by the underlying hardware devices. 
   The incoming data is switched by the switch  141   a - c  to either regular TCP/IP stack  140   a - c  or to the group-to-group communication stack  130   a - c , based on the IP address and/or port. An application process  142  of node  100  communicates using the standard TCP stack  140 . The application segments  135   x,y ,  136   a,y  communicate with group communication stack  130  respectively. The  105  carry control signals that coordinate and controls operations of group  131 . The scope of the signals sent over control bus  105  is limited to the first group  120 . The virtual bus  143  is comprised of the first and the second data streams  110 ,  111  and control signals  105  spanning group  120 . This bus is directly tapped into by the peer group TCP connection  130 . 
   An alternative to the virtual bus  143  is the point to point communication between nodes and has the advantage of better bandwidth usage. However, this necessitates each node in a communication system to keep track of other nodes and their addresses and their roles. In one embodiment, a logical bus model is preferred over control messaging due to location and identity transparency. 
   Referring to  FIG. 2   c  illustrated is the connection end point  130   a  in accordance with one embodiment of the present invention. Generally, the switch  141  directs data to either standard TCP stack or the group-group communication stacks Internet Protocol (“IP”) input  171 . For fragmented IP packets,  170  performs reassembly before passed to  171 . When input packet is not fragmented, it may be passed directly to the input content filter  171  after few basic consistency checks. The input content filter  171  examines the input data content and or packet header to determine if it contains data to be passed in to the application segment (e.g.,  135   x ,  135   y , or  136   x ). 
   If the communication system determines not to pass packet further up, it is discarded with no further action and any memory is freed. Otherwise the input content filter  171  marks segments of the packet that is being passed into application. The packet is then passed to IP input processing layer  172  for complete validation including checksum computation and other consistency checks. Any invalid packets are discarded with no further processing. Resulting packets are then passed into a group-group TCP layer  173 . The group-group TCP layer  173  coordinates with group nodes (e.g.,  120 ,  160 ) and controls data receipt to meet TCP specification requirements such as acknowledgements to peer group. The group-group TCP layer  173  maintains the input TCP states of connection and passes data to socket through  137 . The output data path ( 137 ) and input data path ( 138 ) interfaces with to application socket for connections such as  130 . 
   The user socket sends out data invoking an output content filter  174 . In one embodiment, the output content filter  174  is not installed, and hence, performs no operation. A filter for fault tolerance, synchronously compare data to be sent with other replica segment outputs and transmits a single output instance. The selection of output instance transmitted to peer group depends on the policy set in the filter such as equal outputs, majority agreement, correct result or successful operation output etc. Upon failure of a transmitting segment instance, a replica takes over and continues transmissions without connection disruption. At successful output instance reception at peer group, the replicas discard the data and frees up memory. The output content filter  174  passes data for transmission, to a group TCP output layer  175 . The group TCP output layer  175  controls data transmission and maintain transmission states in conjunction with group nodes. The group TCP output layer  175  works with its group nodes to transmit data to peer group in the sequential order as specified by TCP. The group TCP output layer  175  passes an IP output layer  176  with data to transmit. The IP output layer  176  later performs standard IP functions on the data and passes it down to device driver  177  for data transmission. 
   When an output comparison result by the output content filter  174  indicates differing outputs produced by nodes, a subset replicas are considered faulty and excluded from further service over connection while remaining endpoints continue service without connection disruption. In an embodiment having the exclusion of an endpoint, such exclusion is based on schemes where majority of endpoints agree on a result to exclude others. Alternatively, exclusion of endpoints may occur where an operation failed. Exclusion of an endpoint may also be from any application specific scheme that is programmable with filter. Upon failure of an endpoint during transmission of data, a replica endpoint if any completes the transmission without disruption of the connection. 
   Data Output Over a Connection 
   Multiple end points of a group may transmit data in TCP order. It is thus necessary to assign consecutive sequence numbers to segments of data to be transmitted. It is also necessary to maintain of the consistency of data transmitted, in order to avoid mixing up distinct request/responses from endpoints. For this purpose each complete request/response data is treated as a record by the transmitting node. 
   Node Based Group-to-group Communication 
   Referring to  FIG. 3   a , it is a block diagram of a general computer and its elements suitable for implementing elements of invention. Here the group-to-group communication stack is executed by the processor(s) in the system. 
   Group-to-group Communication Offloading the Main CPU 
   Referring to  FIG. 3   b , it is a block diagram of a computer and its elements suitable for implementing elements of invention while offloading main processor from processing certain elements. The group-group communication stack is offloaded to an adaptor card with it own processor. 
   Group-to-group Communication on Integrated Circuits 
   Referring to  FIG. 3   c , it is a block diagram of a computer and its elements suitable for implementing elements of invention while offloading main processor from processing certain elements of invention to dedicated hardware/accelerator integrated chips. The offloads most of the processing required otherwise by the main CPU by implementing the group-group communication stack fully or partially. 
   Referring to  FIG. 4 , illustrated is a logical view of an implementation where input data is shared as in a bus however the output data is switched. The  400  is the input data stream from the peer group. The  410  is a logical half-bus where only input is shared using multicast or a shared media such as Ethernet. The  420 ,  421  and  422  represent the bus input end-points to the nodes  430 ,  431  and  432  respectively and  440 ,  441  and  442  are the output end points that get fed into a layer  2  or layer  3  IP switching device  450 . The  460  represent the aggregate output produced by the nodes  430 ,  431  and  432  produced for input  400 . The  400  and  460  respectively forms input and output of a single connection. 
   Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs for fault tolerant symmetric multi-computing, in accordance with the disclosed principles of the present invention. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.