Abstract:
A method of Ethernet virtualization using automatic self-configuration of logic of a data router. The method comprising maintaining control parameters at a master device, accessing, by a slave device, the control parameters at the master devices, and configuring the slave device based on the accessed control parameters.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to network virtualization and, more particularly, to methods, systems, computer program products for implementing Ethernet virtualization routers using an automatic self-configuration of logic. 
         [0002]    Current network virtualization techniques in multiple logic device systems require synchronization of common control parameters between them. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    In one embodiment, the present application provides an enhanced technique for Ethernet virtualization that may help overcome the foregoing deficiencies by self-configuration of logic. If one logic device is designated as the master, defined as the device containing the desired configuration values, one or more slaves can configure themselves based on those values. Logic can be implemented that handles auto-configuration as one or more slaves reading from this master. The slave devices can continually (or any periodic basis) check for changes in the configuration settings and update themselves with any such changes. They can also provide reporting and checking of these values. In such a manner, systems with multiple logic devices can easily stay in-sync without external intervention. 
         [0004]    Exemplary embodiments include a method for Ethernet virtualization using an automatic self-configuration of logic. The method comprises maintaining control parameters at a master device, accessing, by a slave device, the control parameters at the master devices, and configuring the slave device based on the accessed control parameters. 
         [0005]    A system and a computer program product corresponding to the above-summarized method is also described and claimed herein. Other systems, methods, computer program products, or hardware products according to embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional systems, methods, computer program products, and/or hardware products be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
         [0006]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0008]      FIGS. 1A and 1B  together comprise a flowchart setting forth an illustrative operational sequence for Ethernet virtualization to facilitate flow of unknown traffic to a virtual host; 
           [0009]      FIG. 2  is a block diagram setting forth an illustrative system for Ethernet virtualization to facilitate flow of unknown traffic to a virtual host; 
           [0010]      FIG. 3  is a data structure diagram setting forth an illustrative receive buffer for use with the system of  FIG. 2 ; 
           [0011]      FIG. 4  is a data structure diagram setting forth an illustrative elastic FIFO for use with the system of  FIG. 2 ; 
           [0012]      FIG. 5  is a block diagram setting forth an illustrative computer program product or hardware product for Ethernet virtualization to facilitate flow of unknown traffic to a virtual host; 
           [0013]      FIG. 6  is a block diagram setting forth an illustrative system for implementing auto self-configuration of logic; 
           [0014]      FIG. 7  is a flowchart illustrating the operational sequence for auto self-configuration of logic; and 
           [0015]      FIG. 8  is a flowchart illustrating the operational sequence for auto self-configuration of logic including data verification and correction. 
       
    
    
       [0016]    The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0017]    An exemplary embodiment of the present invention permits a hardware Ethernet virtualization router which serves a large number of communications channels to maintain high bandwidth by ensuring that packets can continue moving. The router avoids having to place back pressure on the network or having to drop packets, which are undesirable alternatives to moving packets. Thus, the technical effects and benefits include allowing an Ethernet virtualization router servicing any number of communications channels to continually move traffic efficiently regardless of packet types or shortages in channel-specific resources. High bandwidth can be maintained in spite of the unpredictable and bursty traffic patterns inherent to Ethernet networks. Packet order is maintained on a host-by-host basis and does not stall the flow of packets to one virtual machine because of a shortage of resources required for another virtual machine. Packets requiring special attention, such as multicast packets, packets for which connections cannot be determined by a parser, or address resolution protocol (ARP) packets, may also be handled using the methods disclosed herein. 
         [0018]      FIGS. 1A and 1B  together comprise a flowchart setting forth an illustrative operational sequence for Ethernet virtualization to facilitate flow of unknown traffic to a virtual host, and  FIG. 2  is a block diagram setting forth an illustrative system for Ethernet virtualization using an elastic FIFO memory to facilitate flow of unknown traffic to a virtual host. Although  FIGS. 1A and 1B  show a linearly sequential series of operations, this is solely for purposes of clarity and illustration, as some of the steps could be performed in parallel or in a sequence other than what is shown in  FIGS. 1A and 1B . The operational sequence of  FIGS. 1A and 1B  commences at block  101  ( FIG. 1A ) where a packet is received that represents unknown traffic destined for a virtual host. The packet is received from a network. The received packet includes a header. Illustratively, the received packet may be written into a relatively small optional receive buffer  203  ( FIG. 2 ) as the packet arrives from a network adapter  201 . The receive buffer  203  is not strictly required. 
         [0019]    At block  103  ( FIG. 1A ), a parser  205  ( FIG. 2 ) examines each packet&#39;s headers (for example, layer 2, layer 3, etc.) and a first test is performed to ascertain whether or not a destination connection can be determined for the received packet. The affirmative branch from block  103  leads to block  107 , and the negative branch from block  103  leads to block  111  (described in greater detail hereinafter). If the parser  205  ( FIG. 2 ) can determine a destination connection for the packet, following block  107  at block  105  ( FIG. 1A ), a second test is performed to ascertain whether or not one or more connection-specific resources required to send the packet to a virtual host memory corresponding to the destination connection are available. The one or more connection-specific resources are available if they can be obtained for allocation by a resource allocation mechanism such as an allocator  207  ( FIG. 2 ). For example, the allocator  207  attempts to obtain one or more connection-specific resources required to send that packet to a virtual machine  223  that corresponds to the destination connection. Note that the terms “virtual machine” and “virtual host” are used interchangeably herein. The affirmative branch from block  105  leads to block  109  (to be described in greater detail hereinafter), whereas the negative branch from block  105  leads to block  113  (to be described in greater detail hereinafter). 
         [0020]    Although the illustrative configuration of  FIG. 2  shows one virtual machine  223 , this is for purposes of clarity, as the system of  FIG. 2  may contain a plurality of virtual machines. For example, a practical system may include hundreds or thousands of virtual machines. With hundreds or thousands of virtual machines running on a single physical host, there may be scattered shortages of resources for some small subset of the virtual machines at any given time. Within the physical machine, a hypervisor may allocate differing resources to different virtual machines. For example, some virtual machines may have higher priority and therefore get more time-slices than others. Or, some virtual machines may have more physical storage allocated for use as receive buffers. A real-world system hosting thousands of virtual machines may not operate perfectly at all times with regard to resource management. Likewise, scattered resource shortages may be attributed to the inherently bursty nature of Ethernet traffic. 
         [0021]    At block  107  ( FIG. 1A ), a local engine  213  ( FIG. 2 ) blocks the allocator  207  from allocating resources to new packets when packets for the same connection already exist within an elastic first-in, first-out (FIFO)  500  memory because of a previous lack of resources. At block  109  ( FIG. 1A ), if the allocator  207  is successful in obtaining the resources, a send engine  209  ( FIG. 2 ) writes the packet to a virtual host memory  221  associated with the virtual machine  223 . If the parser  205  does not determine the packet&#39;s connection ( FIG. 1A , block  111 ), it passes the packet to a store engine  211  ( FIG. 2 ). At block  113  ( FIG. 1A ), if the allocator  207  ( FIG. 2 ) fails in its attempt to obtain the required resources (which could be because the resources are not available or because the local engine currently has priority access to those resources), the allocator  207  passes the packet to the store engine  211 . 
         [0022]    Next, at block  115  ( FIG. 1A ), for each packet it is to service, the store engine  211  ( FIG. 2 ) obtains a free packet buffer from the elastic FIFO  500 . A free packet buffer is an empty packet-sized block of memory in a local data store  215 . The store engine  211  moves the packet into that buffer ( FIG. 1B , block  117 ) and submits the used buffer to the elastic FIFO  500  ( FIG. 2 ). If a free packet buffer resource is not available, the packet is dropped or, optionally, the store engine  211  can wait for that shared resource to become available provided sufficient buffering, such as the receive buffer, is available. Since a packet&#39;s connection must be determined before it can be sent to the virtual host memory  221 , at block  119  ( FIG. 1B ) an assist engine  217  ( FIG. 2 ) determines and assigns connections to packets that were submitted to the elastic FIFO  500  without known connections (i.e. those packets which arrived from the parser  205 ). 
         [0023]    The procedure of  FIGS. 1A and 1B  progresses to block  121  ( FIG. 1B ) where the local engine  213  ( FIG. 2 ) continuously or periodically or repeatedly monitors both the allocator  207  for connection-specific resources and the elastic FIFO  500  for the presence of packets with known destination connections. When resources become available for a connection which had previously lacked resources, block  129 , the local engine  213  ( FIG. 2 ) gains exclusive access to those resources, via the allocator  207 , until no more packets for that connection exist in the elastic FIFO  500 . The operations of block  129  ( FIG. 1B ) are critical to maintaining packet order within destination connections. At block  123  ( FIG. 1B ), when a connection with both waiting packets and available resources is seen, the packet is removed from the local data store  215  ( FIG. 2 ) and passed to the allocator  207 . The allocator  207  allocates the connection-specific resources required to send that packet to a virtual machine  223  corresponding to the connection destination ( FIG. 1B , block  125 ). Since the local engine  213  ( FIG. 2 ) already determined that the resources were available and claimed them for the packet, the allocator  207  is successful and the packet is written to virtual host memory  221  by the send engine  209  at block  127  ( FIG. 1B ). 
         [0024]    The parser  205  ( FIG. 2 ), allocator  207 , send engine  209 , store engine  211 , local engine  213 , local data store  215 , elastic FIFO  500 , and assist engine  217  together comprise a virtualization router  200 . The router  200  is referred to as a virtualization router because it supports communication channels to a plurality of virtual machines which are called destination connections, such as virtual machine  223 , providing the illusion to each virtual machine  223  that it possesses its own network interface card (such as the network adapter  201 ), when in fact only a single high-speed adapter (i.e., the network adapter  201 ) is present. The network adapter  201  is run in promiscuous mode to receive all packets off the network. The router  200  determines the correct connection for each packet and moves the packet to a memory space (i.e., virtual host memory  221 ) of a corresponding virtual machine  223 . 
         [0025]    Inbound Ethernet traffic flow is inherently bursty. Multiple shared and non-shared resources are required for each connection in order to move its packets. With inherently limited resources which must be carefully managed, one of the functions performed by the router  200  is to handle the bursty traffic in such a way as to minimize packet loss and retransmission for each connection in the context of virtual hosts, thereby maximizing network efficiency. 
         [0026]      FIG. 3  is a data structure diagram setting forth an illustrative receive buffer  401  for implementing the optional receive buffer  203  of  FIG. 2 . Incoming bursts of packet data are initially written directly into the receive buffer  401  ( FIG. 3 ), which is managed with a queue of records called an addrq  403 . Each record  405  contains various information about a burst including a pointer into the receive buffer  401  where the data was written. All packets are removed from the receive buffer  401  in the same order that they were previously written (as the packets arrived from the network adapter  201 ,  FIG. 2 ). If the packet at the head of the addrq  403  ( FIG. 3 ) as indicated by a head pointer  407  is not a type of packet that requires special handling and if all required resources are available to move the packet, the packet is sent directly to its connection destination (virtual host memory  221 ,  FIG. 2 ). Otherwise, if the packet is a special type or if one or more of the resources is not available, the packet is sent to the elastic FIFO  500 . 
         [0027]    As the head pointer  407  ( FIG. 3 ) moves forward (i.e., in a downward direction wherein  FIG. 3  is oriented such that the reference numerals are upright), packets for connections with ample resources continue to be sent directly to their connection destinations even while packets for other connections without resources are sent to the elastic FIFO  500  ( FIG. 2 ). Similarly, by the same mechanism, packets that do not require special consideration can be sent directly to their destinations while packets that require extra processing (e.g. multicast packets) can be sent to the elastic FIFO  500  to be serviced by an independently-running assist engine. In this way, the flow of all packets continues through the router  200  consistently without stalls or hesitations. 
         [0028]      FIG. 4  is a data structure diagram setting forth an illustrative elastic FIFO  500  memory ( FIGS. 2 and 4 ) for use with the system of  FIG. 2  and the method of  FIG. 1 . The elastic FIFO  500  ( FIG. 4 ) includes a large array of linked lists of packet buffers, as well as all the functionality required to maintain those lists and allow them to be put to a useful purpose. The elastic FIFO  500  serves to manage the flow of packets that can not be sent directly from the receive buffer  203  ( FIG. 2 ) to one of the virtual machines such as the virtual machine  223 . The elastic FIFO  500  ( FIG. 4 ) is called “elastic” because it changes shape with changes in either or both of: (a) the number of active connections, or (b) the total number of packets contained by the FIFO. In its entirety, the memory footprint of the elastic FIFO  500  can be increased or decreased over time as free buffers are dynamically added or removed. 
         [0029]    The elastic FIFO  500  is also called “elastic” because it is actually a collection of queues on which packet buffers and pointers to packet buffers are held. These pointers to packet buffers are shown in  FIG. 4  as indirect buffers  505 . i , where i is any positive integer greater than zero. The packet buffers are shown in  FIG. 4  as normal buffers  507 . j  where j is any positive integer greater than zero. There are named queues  501  for free normal buffers  507 . j  of various sizes. These named queues  501  also include a named queue for free indirect buffers  505 . i . Indirect buffers  505 . i  only occupy a small amount of memory, as they merely point to a normal buffer  507 . j , but these indirect buffers  505 . i  do occupy some memory and are thus obtained as needed from the free list so that they may be enqueued to a numbered list. Since the elastic FIFO  500  is elastic, additional free indirect buffers  505 . i  can be added as necessary, and these buffers may also be taken away if the free list becomes excessively long. There is no direct relationship between the total number of indirect buffers  505 . i  and normal buffers  507 . j . At any given time, some number of the normal buffers  507 . j  will be occupied by packets, though ideally most should be free most of the time. 
         [0030]    There are two categories of queues within the elastic FIFO  500 : named queues  501  and numbered queues  503 . When a packet is sent to the elastic FIFO  500  because it requires special consideration, it is placed on a named queue of named queues  501 , as opposed to being placed on a numbered connection queue of numbered queues  503 . Each named queue of named queues  501  includes a list name A, B, C, D, Z that reflects a special packet type placed on that queue. Packets placed on named queues  501  must ultimately be moved to numbered queues  503 . Packets on a numbered queue of numbered queues  503  can be sent to a corresponding destination connection&#39;s virtual host memory  221  ( FIG. 2 ) as soon as the required connection-specific resources are available. 
         [0031]    Named queues  501  ( FIG. 4 ) are also used for implementing pools of various types of free buffers, which are buffers that are not currently associated with a packet. A free buffer is obtained for each packet that is to be locally stored, and that buffer is returned to the pool from which it originated once the packet has been removed from local storage and sent to virtual host memory  221  ( FIG. 2 ). When the special packet type is multicast or broadcast, the packet must be moved from a “multicast” or “broadcast” named queue of named queues  501  ( FIG. 4 ) to a plurality of numbered queues in numbered queues  503  so it may be sent to multiple virtual machines including virtual machine  223  ( FIG. 2 ), wherein potentially thousands of such virtual machines are present. The assist engine  217  efficiently performs this task through the use of indirect buffers  505 . i  ( FIG. 4 ). 
         [0032]    There are two types of packet buffers that may be put on a numbered queue of numbered queues  503 : normal buffers  507 . j  and indirect buffers  505 . i . Indirect buffers  505 . i  do not contain data themselves, but merely point to a normal buffer  507 . j . The assist engine  217  ( FIG. 2 ) removes a normal buffer  507 . j  ( FIG. 4 ) from a named queue of named queues  501 , obtains multiple free indirect buffers  505 . i  from the elastic FIFO  500 , points those indirect buffers  505 . i  at the normal buffer  507 . j , and enqueues those indirect buffers  505 . i  to the appropriate numbered queues  503 . 
         [0033]    Every normal buffer  507 . j  carries a use count  509 . A normal buffer&#39;s use count  509  is usually 1 but can be higher when the normal buffer  507 . j  is the target of an indirect buffer  505 . i . A normal buffer  507 . j  that is directly sitting on a queue has a use count of 1, while a normal buffer  507 . j  pointed to by one or more indirect buffers  505 . i  (which are sitting on one or more queues) has a use count equal to the number of indirect buffers  505 . i  pointing to it. A normal buffer  507 . j  that is the target of an indirect buffer  505 . i  can not itself directly exist on any queue. Each time a copy of the packet in the normal buffer  507 . j  is sent to virtual host memory  221  ( FIG. 2 ), an indirect buffer  505 . i  ( FIG. 4 ) pointing to it is removed from a numbered queue of numbered queues  503  and the normal buffer&#39;s use count  509  is decremented by 1 (provided it is still greater than 1). When the normal buffer&#39;s use count  509  reaches 1, it is returned to the pool of free normal buffers  507 . j  at the same time a final indirect buffer  505 . i  pointing to the normal buffer  507 . j  is dequeued and returned to the pool of free indirect buffers  505 . i.    
         [0034]    The local engine  213  ( FIG. 2 ) performs the task of dequeuing buffers from the numbered queues  503  ( FIG. 4 ), via the interface provided by the elastic FIFO  500 , so the packets contained within or pointed to by those buffers can be sent to virtual host memory  221  ( FIG. 2 ). As soon as the allocator  207  fails to obtain virtual host memory  221  for one packet for a given connection, it must send all subsequent packets for that connection to the local data store  215  in order to maintain packet order for that connection. One task performed by the local engine  213  is to empty queues containing packets that have accumulated, due to resource shortages, so packets may once again flow directly from the network adapter  201  to virtual host memory  221 , i.e. without being stored first. The local engine  213  obtains exclusive access to connection-specific resources until the local engine determines that it has emptied a destination connection&#39;s queue and relinquishes that exclusivity. 
         [0035]      FIG. 5  is a block diagram setting forth an illustrative computer program product or hardware product for Ethernet virtualization using an elastic FIFO memory to facilitate flow of unknown traffic to a virtual host. The system includes a computer  300  operatively coupled to a signal bearing medium  340  via an input/output interface (I/O)  330 . The signal bearing medium  340  may include a representation of instructions for Ethernet virtualization using an elastic FIFO memory to facilitate flow of unknown traffic to a virtual host, and may be implemented as, e.g., information permanently stored on non-writeable storage media (e.g., read-only memory devices within a computer, such as CD-ROM disks readable by a CD-ROM drive), alterable information stored on a writeable storage media (e.g., floppy disks within a diskette drive or hard disk drive), information conveyed to a computer by a communications medium, such as through a computer or telephone network, including wireless or broadband communications networks, such as the Internet, etc. 
         [0036]    The computer  300  includes a processor  310  that processes information for Ethernet virtualization using an elastic FIFO memory to facilitate flow of unknown traffic to a virtual host, wherein the information is represented, e.g., on the signal bearing medium  340  and communicated to the computer  300  via the I/O  330 , wherein the processor  310  saves information as appropriate into a memory  320 . Illustratively, the processor  310  corresponds to the processing mechanism  106  of  FIG. 1 . Returning now to  FIG. 6 , this information may also be saved into the memory  320 , e.g., via communication with the I/O  330  and the signal bearing medium  340 . 
         [0037]    The processor  310  executes a program for implementing Ethernet virtualization to facilitate flow of unknown traffic to a virtual host. The processor  310  implements instructions for receiving a packet that represents unknown traffic destined for a virtual host on a network; performing a first test to ascertain whether or not a destination connection can be determined for the received packet wherein, if a destination connection can be determined, a second test is performed to ascertain whether or not one or more connection-specific resources required to send the packet to a virtual host memory corresponding to the destination connection are available; if a destination connection for the packet cannot be determined, passing the packet to a store engine; if the one or more connection-specific resources are not available, passing the packet to the store engine; the store engine obtaining a free packet buffer from an elastic first-in, first-out (FIFO) memory, wherein the free packet buffer is an empty packet-sized block of memory in a local data store; the store engine moving the packet into the free packet buffer and submitting the free packet buffer to the elastic FIFO memory; performing a monitoring procedure to detect both an availability of connection-specific resources and a presence of one or more waiting packets with a known destination connection; when a destination connection with: (i) one or more waiting packets, and (ii) available connection-specific resources; are both detected, removing the packet from the local data store; allocating the one or more connection-specific resources required to send the packet to the virtual host memory corresponding to the connection destination; and writing the packet to the virtual host memory. The foregoing steps may be implemented as a program or sequence of instructions within the memory  320 , or on a signal bearing medium, such as the medium  340 , and executed by the processor  310 . 
         [0038]      FIG. 6  shows a system  600  for automatically configuring a plurality of client devices from a master record located as a master device in accordance with an embodiment of the present invention. In one embodiment, the router  200  ( FIG. 2 ) may be implemented as a plurality of hardware components. It is important to keep the configuration of the components synchronized for proper operation. The configuration of these components can be managed by automatic self-configuration of logic as disclosed in the present application. The system  600  of  FIG. 6  can be used to automatically configure client devices from a master record. To illustrate, client devices  604 ,  606  have an interface to provide access to the master data copy from the master device  602 . 
         [0039]    In one embodiment, the interface comprise a read—write bus configuration allowing each client  604 ,  606  to read the master device  602 . Data writes may also be performed for status updates or other uses. Client  604  or  606  raises a request line with an accompanying address and data size count, which targets a resource within the master copy  602 . Since multiple devices may simultaneously request access to the same master copy  602 , the targeted device must do internal arbitration and respond to each request separately. To complete the request, the master copy  602  responds with data and an acknowledgement signal, and the client  604  or  606  drops the request line. A write is performed in a similar manner, except the client also provides data at the time the request is initiated, and the master copy responds only with an acknowledgement and no data. The client devices  604 ,  606  include auto-configuration logic  601  configured to allow the client devices to execute automatic self-configuration function. For example, such function can include configuration logic  700  of  FIG. 7  and/or circular data checking logic  800  of  FIG. 8 , as explained below in detail. The client devices are typically implemented on an external programmable logic device, such as field programmable gate array (FPGA) and the master record is stored within another programmable logic device such as an application specific integrate circuit (ASIC). 
         [0040]    In one embodiment, an Ethernet router that supports virtualization will scale to support multiple assistive devices simultaneously. That is, multiple parallel engines can be formed and tied together. In particular, engines such as  205  the packet parser might be duplicated or put into a programmable device such as  604 ,  606 . All of these multiple engines need to be kept in synchronization with the master copy  602  of configuration data. This helps ensure consistency of operation within the virtualized router depicted in  FIG. 2 . 
         [0041]      FIG. 7  is a flow chart  700  illustrating the operational sequence for automatic self-configuration of logic in accordance with an embodiment of the present application. The operational sequence can be implemented by the system  600  of  FIG. 6 . At block  702 , a client device, such as device  604  ( FIG. 6 ) begins execution of the automatic configuration function  610 . At block  704 , the client device checks if it is not already configured, or reconfiguration is desired. If it is not already configured, or reconfiguration, then, at block  706 , the client device begins to read the master data  608  from the master device  602 , via the interface as explained above. At block  708 , when the master data arrives at the client device, it stores the master data in its memory or other storage. At block  710 , the client device checks if there is additional data to be read. If there is additional data to be read, then the configuration of the client device is not complete and client device returns to block  706  to read additional master data and then to block  708  to store the read data. Once configuration is complete, the read—store loop exits and ends processing, at block  712 . 
         [0042]      FIG. 8  is a flow chart  800  illustrating the operational sequence for auto self-configuration of logic including data verification and correction in accordance with an embodiment of the present application. The flow chart  800  shows a circular data checking—reconfiguring loop which can be used with the system  600  of  FIG. 6 . At block  802 , the client device checks the accuracy of the configuration data stored in the device. For example, the client device can check parity error or other failed check. If there is a data error, this event triggers the client device to proceed to block  804  to reconfigure itself and replace the damaged data. Then the client device proceeds to block  806  which include causing the client device to store the new data and check the data for accuracy. In this manner, the client device resumes normal operation. 
         [0043]    It should be noted that the above techniques can be used with the computer system  300  and signal bearing medium  340  of  FIG. 5 . 
         [0044]    As described above, the embodiments of the invention may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. Embodiments of the invention may also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. 
         [0045]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.