Abstract:
A system and method that modifies the behavior of the IEEE 802.1D STP standard to thereby decouple the one data domain from the one control domain involves managing multiple spanning tree protocol (STP) instances in a virtual local area network (VLAN). The method includes the step of assigning a unique set of ports within the VLAN to each of the multiple STP instances. Then, each of the multiple STP instances are managed to keep each of the multiple STP instances separate. Finally, when a topology change is detected in one of the multiple STP instances, entries that have been learned on the unique set of ports assigned to the STP protocol instance where the topology change is detected are fast-aged or transitioned from one state to another.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention disclosed herein relates generally to network configuration protocols. More specifically, the invention relates to the standard IEEE 802.1D spanning tree protocol in virtual local area networks. 
     2. Related Art 
     A computer network typically comprises a plurality of interconnected entities. An entity may consist of any network device, such as a server or end station, that transmits or receives data frames. A common type of computer network is a local area network (“LAN”) which typically refers to a privately owned network within a single building or campus. LANs typically employ a data communication protocol, such as Ethernet or token ring, that defines the functions performed by the data link and physical layers of a communications architecture. In many instances, several LANs are interconnected by point-to-point links, microwave transceivers, satellite hook-ups, etc. to form a wide area network (“WAN”) or intranet that may span an entire country or continent. 
     One or more intermediate network devices are often used to couple LANs together and allow the corresponding entities to exchange information. For example, a bridge may be used to provide a bridging function between two or more LANs. Alternatively, a switch may be utilized to provide a switching function for transferring information between a plurality of LANs or end stations. Bridges and switches are devices that operate at the Data Link layer (“layer  2 ”) of the Open Systems Interconnection (“OSI”) model. Their operation is defined in the American National Standards Institute (“ANSI”) Institute of Electrical and Electronics Engineers (“IEEE”) 802.1D standard. A copy of the ANSI/IEEE Standard 802.1D, 1998 Edition, is incorporated by referenced herein in its entirety. 
     Typically, a switch (or bridge) is a computer that includes a plurality of ports that are coupled to the LANs or end stations. Ports used to couple switches to each other are generally referred to as trunk ports. Ports used to couple switches to LANs or end stations are generally referred to as access ports. The switching function includes receiving data from a sending entity at a source port and transferring that data to at least one destination port for forwarding to a receiving entity. 
     Switches typically learn which destination port to use in order to reach a particular entity by noting on which source port the last message originating from that entity was received. This information is then stored in a block of memory referred to as a filtering database. Thereafter, when a message addressed to a given entity is received on a source port, the switch looks up the entity in its filtering database and identifies the appropriate destination port to reach that entity. If no destination port is identified in the filtering database, the switch floods the message out all ports, except the port on which the message was received. Messages addressed to broadcast or multicast addresses are also flooded. 
     A computer network may be segregated into a series of logical network segments. For example, any number of physical ports of a particular switch may be associated with any number of groups within the switch by using a virtual local area network (“VLAN”) arrangement that virtually associates the port with a particular VLAN designation. 
     The VLAN designation for each local port is stored in a memory portion of the switch such that every time a message is received by the switch on a local port the VLAN designation of that port is associated with the message. Association is accomplished by a flow processing element which looks up the VLAN designation in the memory portion based on the local port where the message originated. 
     Most computer networks include redundant communications paths so that a failure of any given link or device does not isolate any portion of the network. The existence of redundant links, however, may cause the formation of loops within the network. Loops are highly undesirable because data frames may traverse the loops indefinitely. Furthermore, because switches and bridges replicate (i.e., flood) frames whose destination port is unknown or which are directed to broadcast or multicast addresses, the existence of loops may cause a proliferation of data frames that effectively overwhelms the network. 
     To avoid the formation of loops, many intermediate network devices execute a spanning tree algorithm that allows them to calculate an active network topology which is loop-free and yet connects every pair of VLANs within the network. The IEEE 802.1D standard defines a spanning tree protocol (“STP”) to be executed by 802.1D compatible devices (e.g., bridges, switches, and so forth). With the IEEE 802.1D STP standard, one data domain is coupled with one control domain. 
     In general, by executing the STP, switches elect a single switch to be the root bridge. In addition, for each VLAN coupled to more than one switch, only one (the designated bridge) is elected to forward frames to and from the respective VLAN. The designated bridge is typically the one closest to the root. Each bridge also selects one port (its “root port”) which gives the lowest cost path to the root. 
     The root ports and designated bridge ports are selected for inclusion in the active topology and are placed in a forwarding state so that data frames may be forwarded to and from these ports and thus onto the corresponding paths or links of the network. Ports not included within the active topology are placed in a blocking state. When a port is in the blocking state, data frames are not forwarded to or received from the port. A network administrator may also exclude a port from the spanning tree by placing it in a disabled state. 
     To obtain the information necessary to run the STP, switches exchange special control messages called bridge protocol data unit (“BPDU”) messages. Conventional BPDU messages contain a number of fields, including a root bridge ID (“BID”) which is the current root bridge; a path cost to the root bridge which indicates the distance to the root bridge; a sender BID which is the BID of the switch that sends the BPDU; and a port ID which is the actual port on the switch that the BPDU was sent from. 
     All of the switches constantly send BPDUs to each other, trying to determine the best path between various segments. When a switch receives a BPDU (from another switch) that is better than the one it is broadcasting for the same segment, it will stop broadcasting its BPDU out that segment. Instead, it will store the other switch&#39;s BPDU for reference and for broadcasting out to segments that are farther away from the root bridge. 
     A root bridge is chosen based on the results of the BPDU process between the switches. Initially, every switch considers itself the root bridge. When a switch first powers up on the network, it sends out a BPDU with its own BID as the root BID. When the other switches receive the BPDU, they compare the BID to the one they already have stored as the root BID. If the new root BID has a lower value, they replace the saved one. But if the saved root BID is lower, a BPDU is sent to the new switch with this BID as the root BID. When the new switch receives the BPDU, it realizes that it is not the root bridge and replaces the root BID in its table with the one it just received. The result is that the switch that has the lowest BID is elected by the other switches as the root bridge. 
     Based on the location of the root bridge, the other switches determine which of their ports has the lowest path cost to the root bridge. These ports are called root ports, and each switch (other than the current root bridge) must have one. 
     The switches determine who will have designated ports. A designated port is the connection used to send and receive packets on a specific segment. By having only one designated port per segment, all looping issues are resolved. 
     Designated ports are selected based on the lowest path cost to the root bridge for a segment. Since the root bridge will have a path cost of “0,” any ports on it that are connected to segments will become designated ports. For the other switches, the path cost is compared for a given segment. If one port is determined to have a lower path cost, it becomes the designated port for that segment. If two or more ports have the same path cost, then the switch with the lowest BID is chosen. 
     Once the designated port for a network segment has been chosen, any other ports that connect to that segment become non-designated ports. These non-designated ports block network traffic from taking that path so it can only access that segment through the designated port. 
     Each switch has a table of BPDUs that it continually updates. The network is now configured as a single spanning tree, with the root bridge as the trunk and all the other switches as branches. Each switch communicates with the root bridge through the root ports, and with each segment through designated ports, thereby maintaining a loop-free network. 
     In response to network changes or failures, BPDU information is up-dated, and/or it times-out and causes the active spanning tree topology to be re-calculated. As a result, ports may transition from the blocking state to the forwarding state and vice versa. When a topology change is detected, the IEEE 802.1D STP standard moves the ports into fast-aging mode. This means that the Media Control Access (“MAC”) addresses learned on those ports age (or transition from one state to another) at a faster rate (5 times) than normal MAC aging. That is, as a result of new BPDU information, a previously blocked port may learn that it should be in the forwarding state (e.g., it is now the root port or a designated port). Rather than transition directly from the blocking state to the forwarding state, ports typically transition through two intermediate states: a listening state and a learning state. In the listening state, a port waits for information indicating that it should return to the blocking state. If, by the end of a preset time, no such information is received, the port transitions to the learning state. At the end of a second preset time, the port transitions from the learning state to the forwarding state, thereby allowing data frames to be forwarded to and from the port. 
     As mentioned above, the IEEE 802.1D STP standard maintains one control domain. This happens because the VLAN to STP instance is unique. A port in the VLAN is automatically included in the single STP instance associated to that VLAN (there is a one-to-one mapping). Thus, the IEEE 802.1D STP standard suffers from several limitations when implemented in multiple large interconnected networks. The standard STP is prone to slow convergence times, sometimes upward of 30 to 50 seconds, and does not scale well as a topology expands to include additional spanning tree nodes. Additionally, the spanning tree domain (or control domain) must be continuous in order to ensure a loop free data path—changes within the spanning tree domain can affect all spanning tree members of that domain. Such ripple effects, for example, can cause problems in one city to affect other cites where large metro ring topologies are implemented. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system and method that allow a core network to host a plurality of edge (or customer) networks, where each network implements distinct instances of the spanning tree protocol (multiple control domains). The present invention is a system and method that modifies the behavior of the IEEE 802.1D STP standard to decouple the one data domain from the one control domain. 
     A method of the present invention for decoupling the one data domain from the one control domain involves managing multiple spanning tree protocol (STP) instances in a virtual local area network (VLAN). It includes the step of configuring each of the multiple STP instances by assigning unique set of ports within the VLAN to each of the multiple STP instances. Then, each of the multiple STP instances are managed to keep each of the multiple STP instances separate. Finally, when a topology change is detected in one of the multiple STP instances, entries that have been learned on the unique set of ports assigned to the STP protocol instance where the topology change is detected, are fast-aged (or transitioned from one state to another). 
     In the present invention, the step of configuring each of the multiple STP instances by assigning a unique set of ports within the VLAN to each of the multiple STP instances includes associating each of the multiple STP instances with a unique set of ports and an ID for the VLAN. Then, each port in the unique set of ports is associated with its associated STP instance. Finally, a software table is configured for each of the multiple STP instances. 
     Also in the present invention, the step of managing each of the multiple STP instances to keep each of the multiple STP instances separate includes receiving a bridge protocol data unit (BPDU) on a port in the VLAN. Then, which one of the multiple STP instances should process the BPDU is determined. Finally, the BPDU is forwarded to the determined STP instance for processing. 
     The present invention also includes a system for managing multiple STP instances in a VLAN. The system includes multiple STP instances in the VLAN and a STP module. The STP module configures each of the multiple STP instances by assigning a unique set of ports within the VLAN to each of the multiple STP instances. The STP module also manages each of the multiple STP instances to keep each of the multiple STP instances separate. When a topology change is detected in one of the multiple STP instances, the STP module fast-ages entries (or transitions them from one state to another) that have been learned on the unique set of ports assigned to the STP protocol instance where the topology change is detected. 
     The STP module of the present invention configures each of the multiple STP instances by associating each of the multiple STP instances with a unique set of ports and an ID for the VLAN. The STP module then associates each port in the unique sets of ports with its associated STP instance. Finally, the STP module configures a software table for each of the multiple STP instances. 
     The STP module of the present invention manages each of the multiple STP instances to keep each of the multiple STP instances separate by receiving a bridge protocol data unit (BPDU) on a port in the VLAN. The STP module then determines which one of the multiple STP instances should process the BPDU, and forwards the BPDU to the determined STP instance for processing. 
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The present invention will be described with reference to the accompanying drawings, wherein: 
       FIG. 1  is a block diagram representing an example operating environment according to an embodiment of the present invention. 
       FIG. 2  illustrates an example STP instance configuration for the VLAN illustrated in  FIG. 1  according to an embodiment of the present invention. 
       FIG. 3  illustrates an example assignment of ports in the core access device to STP instances illustrated in  FIG. 2  according to an embodiment of the present invention. 
       FIG. 4  illustrates exemplary software tables utilized by the STP instances according to an embodiment of the present invention. 
       FIG. 5  is a flowchart illustrating the high level operation of the present invention according to an embodiment. 
       FIG. 6  illustrates a more detailed operation of the present invention where separate STP instances are configured on one VLAN according to an embodiment. 
       FIG. 7  illustrates a more detailed operation of the present invention where the separate STP instances are managed in the VLAN. 
       FIG. 8  illustrates an example computer that may be used to implement the access devices according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Overview of the Invention 
     With the IEEE 802.1D STP standard, a VLAN has no control plane isolation. The present invention modifies the IEEE 802.1D STP standard so that instead of having a single STP domain (and thus no control plane isolation) running on the VLAN, there are multiple STP domains running on the VLAN. The present invention implements multiple STP domains by supporting multiple STP instances on the VLAN. The support of multiple STP instances by the present invention is accomplished via an STP module. 
     B. System Architecture Overview 
       FIG. 1  is a block diagram representing an example operating environment of the present invention. It should be understood that the example operating environment in  FIG. 1  is shown for illustrative purposes only and does not limit the invention. Other implementations of the operating environment described herein will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein, and the invention is directed to such other implementations. 
     Referring to  FIG. 1 , a layer  2  VLAN  100  is shown. VLAN  100  includes a core network  102 , a core access device  104 , an edge network  106 , an edge access device  108 , an edge network  110  and an edge access device  112 . Core access device  104  includes a STP module  114 . 
     Core access device  104  is connected to edge access device  108  via communication paths  120  and  122 . Core access device  104  is also connected to edge access device  112  via communication paths  116  and  118 . Each of these components will be briefly described next. 
     Core access device  104 , edge access device  108  and edge access device  112  are all 802.1D compatible devices (e.g., bridges, switches, and so forth). Core access device  104 , edge access device  108  and edge access device  112  are end stations to core network  102 , edge network  106  and edge network  110 , respectively. Access devices  104 ,  108  and  112  may each be implemented as a computer that has multiple ports. Communication paths  116 ,  118 ,  120  and  122  provide redundant communications paths so that a failure of any given link or device does not isolate any portion of VLAN  100 . 
     In an embodiment of the present invention, core network  102 , edge network  106  and edge network  110  are all layer  2  networks that make up VLAN  100 . All of the data traffic from core network  102 , edge network  106  and edge network  110  are running on VLAN  100 . Thus, there is no data plane isolation in VLAN  100 . With the IEEE 802.1D STP standard, VLAN  100  would also have no control plane isolation. The present invention modifies the IEEE 802. ID STP standard so that instead of having a single STP domain (and thus no control plane isolation) running on VLAN  100 , there are multiple STP domains running on VLAN  100 . The present invention implements multiple STP domains by supporting multiple STP instances on VLAN  100 . The support of multiple STP instances by the present invention is accomplished via STP module  114 . 
     In an embodiment of the present invention, STP module  114  is a software entity that manages all STP instances of VLAN  100 . STP module  114  may be implemented as an independent module that can run in a single process. For each STP instance it manages, STP module  114  associates with it a unique set of ports in VLAN  100  and VLAN  100 &#39;s ID. Thus, the present invention allows multiple STP instances to be created with the same VLAN ID, each with different ports. The present invention ensures that no STP instances share any ports in VLAN  100 . 
     It follows that each port associated with VLAN  100  is also associated with a specific STP instance. When a BPDU is received on a specific port, STP module  114  looks up the appropriate STP instance that needs to process the BPDU. Once the instance is located, the BPDU is passed onto the appropriate STP instance. The STP instance then processes the BPDU. 
     In addition, each STP instance generates BPDUs only on the ports of which it is associated. Each STP instance controls the port states (e.g., blocking, forwarding, etc.) for its assigned ports on VLAN  100 . This way the two or more STP instances (or domains) associated with VLAN  100  will be completely isolated from each other. When a topology change is detected in VLAN  100 , the IEEE 802.1D STP standard would move the ports into fast-aging mode. This means that the MAC addresses learned on those ports get aged (or transitioned from one state to another) at a faster rate (e.g., 5 times) than normal MAC aging. The same holds true for the present invention. A benefit that the present invention provides is that each STP instance only needs to fast-age entries that have been learned on the ports that it controls. 
     The implementation overhead of multiple STP instances versus a single STP instance is minor. Only three additional bridge timers are needed for each STP instance. No additional port timers are required to implement the present invention.  FIGS. 2 and 3  describe an example configuration of STP instances for VLAN  100 . This example configuration is not meant to limit the invention and is provided for illustrations purposes only. 
     Via STP module  114 , core access device  104  can operate one STP instance for its core network  102 , one STP instance for edge network  106  and one STP instance for edge network  110 . This allows for complete separation between the core and edge STP instances, and thus between the core and edge STP control. In order to achieve STP instance separation, each STP instance running on VLAN  100  must contain a unique set of ports. This is necessary to prevent any ambiguity in the forwarding state of the port within VLAN  100 . This also allows for clear separation between edge network domains and the core network domain. An example STP instance configuration for VLAN  100  is described next with reference to  FIG. 2  and STP module  114 . 
     Referring to  FIG. 2 , STP module  114  includes a STP BPDU demultiplexer  202 , a STP instance  204 , a STP instance  206  and a STP instance  208 . STP instance  204  represents the STP domain that includes a dedicated number of ports on core access device  104  and all of the other device ports in core network  102 . Likewise, STP instance  206  represents the STP domain that includes a dedicated number of ports on core access device  104  and all of the other device ports in edge network  106 . STP instance  208  represents the STP domain that includes a dedicated number of ports on core access device  104  and all of the other device ports in edge network  110 . 
     In an embodiment of the present invention, each of STP instances  204 - 208  keeps track of VLAN  100  and the ports it is controlling by means of a software table. In order for each STP instance  204 - 208  to run independently from the other instances, each instance has its own timers and processes its own BPDUs (i.e., the BPDUs received on any of the ports on VLAN  100  that the STP instance controls). 
     STP BPDU demultiplexer  202  is a module that examines all of the BPDUs received by the ports in VLAN  100 . For each BPDU received, STP BPDU demultiplexer  202  examines its contents and first determines whether it is correct (e.g., no corruption). If the BPDU is correct, then STP BPDU demultiplexer  202  gives the BPDU to the correct STP instance based on the port/VLAN pair from where it was received. 
     The way in which the present invention differentiates BPDUs from one network to another (i.e., from one STP instance to another) is based on the port/VLAN number. This combination is unique. A port/VLAN pair will be controlled by one and only one STP instance. An example assignment of ports in core access device  104  to STP instances  204 - 208  is described next with reference to  FIG. 3 . 
     Referring to  FIG. 3 , ports  1  and  2  are assigned strictly to core network  102  (communication paths not shown). Ports  3  and  4  are assigned strictly to edge network  106 . Specifically, ports  3  and  4  are assigned to communication paths  120  and  122  of edge network  106 , respectively. Ports  5  and  6  are assigned strictly to edge network  110 . Specifically, ports  5  and  6  are assigned to communication paths  116  and  118  of edge network  110 , respectively. 
     This assignment of the ports of core access device  104  may be done via the network administrator of core network  102 . A port should not receive STP BPDUs associated with different STP instances. In the case where the ports of core access device  104  are incorrectly configured by associating different STP instances with the same edge network, a broadcast storm could result. As described above, each of STP instances  204 - 208  may keep track of VLAN  100  and the ports it is controlling by means of a software table. Exemplary software tables based on the port assignment of  FIG. 3  are described next with reference to  FIG. 4 . 
       FIG. 4  illustrates three exemplary software tables, including software table  402 , software table  404  and software table  406 . Software table  402  is maintained by STP instance  204  and illustrates that STP instance  204  controls ports  1  and  2  of core access device  104  in VLAN  100 . Likewise, software table  404  is maintained by STP instance  206  and illustrates that STP instance  206  controls ports  3  and  4  of core access device  104  in VLAN  100 . Finally, software table  406  is maintained by STP instance  208  and illustrates that STP instance  208  controls ports  5  and  6  of core access device  104  in VLAN  100 . Note that each of the software tables  402 - 406  would also include entries for all of the other ports they control. For example, STP instance  204  represents the STP domain that includes a dedicated number of ports on core access device  104  (i.e., ports  1  and  2 ) and all of the other device ports in core network  102 . Therefore, software table  402  would include entries for the other device ports in core network  102 . An exemplary operation of the present invention is described next with reference to  FIGS. 5-7 . 
     C. Operation of the Present Invention 
     The flowchart in  FIG. 5  illustrates the high level operation of the present invention. The flowchart in  FIG. 5  starts at step  502 , where separate STP instances are configured on one VLAN. This step is further described below with reference to  FIG. 6 . 
     In step  504 , the separate STP instances are managed in the VLAN. This step is further described below with reference to  FIG. 7 . 
     In step  506 , when a topology change is detected in one of the STP instances, fast-age only the entries that have been learned on the ports in the VLAN that are associated with the STP instance where the topology change occurred. 
       FIG. 6  further illustrates step  502  in  FIG. 5  regarding configuring separate STP instances on the VLAN. The flowchart in  FIG. 6  starts at step  602 , where STP module  114  configures multiple STP instances by associating each STP instance with a unique set of ports and the VLAN ID. 
     In step  604 , each port in the VLAN is associated with its associated STP instance. This association of the ports with its STP instance may be done by the network administrator. 
     In step  606 , a software table is configured for each STP instance. Note that each software table includes an entry for each port its STP instance controls in the VLAN. 
       FIG. 7  further illustrates step  504  in  FIG. 5  regarding managing the separate STP instances in the VLAN. The flowchart in  FIG. 7  starts at step  702 , where a BPDU is received on a port in the VLAN. 
     In step  704 , STP BPDU demultiplexer  202  examines the BPDU&#39;s contents to ensure they are correct (e.g., they are not corrupted). 
     In step  706 , if the BPDU&#39;s contents are correct, then it is determined whether an STP instance is associated with the port. If the outcome to step  706  is negative, then control passes to step  708 . Alternatively, if the outcome in step  706  is positive, then control passes to step  714 . 
     In step  708 , it is determined whether an STP instance is associated with the VLAN. If the outcome to step  708  is negative, then control passes to step  710  where the BPDU gets forwarded on the VLAN. Alternatively, if the outcome in step  708  is positive, then control passes to step  712  where the BPDU gets dropped. 
     In step  714 , STP BPDU demultiplexer  202  determines the correct STP instance to process the BPDU based on the port/VLAN pair from which it was received. 
     In step  716 , STP BPDU demultiplexer  202  provides the BPDU to the determined STP instance to be processed. The BPDU is processed in a similar manner as it is done with the IEEE 802.1D STP standard. An example environment of the present invention is described next. 
     D. Example Environment of the Present Invention 
     Access devices  104 ,  108  and  112  may be implemented using computer system  800  as shown in  FIG. 8 . The present invention may be implemented using hardware, software or a combination thereof and may be implemented in a computer system or other processing system. In fact, in one embodiment, the invention is directed towards one or more computer systems capable of carrying out the functionality described herein. The computer system  800  includes one or more processors, such as processor  804 . The processor  804  is connected to a communication bus  806 . Various software embodiments are described in terms of this example computer system. After reading this description, it will become apparent to a person skilled in the relevant art how to implement the invention using other computer systems and/or computer architectures. 
     Computer system  800  also includes a main memory  808 , preferably random access memory (RAM), and can also include a secondary memory  810 . The secondary memory  810  can include, for example, a hard disk drive  812  and/or a removable storage drive  814 , representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive  814  reads from and/or writes to a removable storage unit  818  in a well known manner. Removable storage unit  818 , represents a floppy disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive  814 . As will be appreciated, the removable storage unit  818  includes a computer usable storage medium having stored therein computer software and/or data. 
     In alternative embodiments, secondary memory  810  may include other similar means for allowing computer programs or other instructions to be loaded into computer system  800 . Such means can include, for example, a removable storage unit  822  and an interface  820 . Examples of such can include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units  822  and interfaces  820  which allow software and data to be transferred from the removable storage unit  818  to computer system  800 . 
     Computer system  800  can also include a communications interface  824 . Communications interface  824  allows software and data to be transferred between computer system  800  and external devices. Examples of communications interface  824  can include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, etc. Software and data transferred via communications interface  824  are in the form of signals which can be electronic, electromagnetic, optical or other signals capable of being received by communications interface  824 . These signals  826  are provided to communications interface via a channel  828 . This channel  828  carries signals  826  and can be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link and other communications channels. 
     In this document, the terms “computer program medium” and “computer usable medium” are used to generally refer to media such as removable storage unit  818 , a hard disk installed in hard disk drive  812 , and signals  826 . These computer program products are means for providing software to computer system  800 . 
     Computer programs (also called computer control logic) are stored in main memory and/or secondary memory  810 . Computer programs can also be received via communications interface  824 . Such computer programs, when executed, enable the computer system  800  to perform the features of the present invention as discussed herein. In particular, the computer programs, when executed, enable the processor  804  to perform the features of the present invention. Accordingly, such computer programs represent controllers of the computer system  800 . 
     In an embodiment where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system  800  using removable storage drive  814 , hard disk drive  812  or communications interface  824 . The control logic (software), when executed by the processor  804 , causes the processor  804  to perform the functions of the invention as described herein. 
     In another embodiment, the invention is implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (ASICs). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s). In yet another embodiment, the invention is implemented using a combination of both hardware and software. 
     E. CONCLUSION 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. This is especially true in light of technology and terms within the relevant art(s) that may be later developed. Thus, the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.