Abstract:
A method and system for reading and propagating accurate time in a worldwide enterprise system through a hierarchical network of time servers, with a single time source at the top of the pyramid, and time source clients at the base. Single source time synchronization is assured by starting with an atomic clock receiver connected to a time source computer. Primary time servers next in the hierarchy access the source to obtain accurate time information. Clients access higher time servers to obtain time information. Time servers operate as multiple hierarchical layers between the clients and the time source, limiting the load on any single server and providing redundant paths to a valid time source. Time servers unable to verify their time through the primary, alternate, or failover servers invalidate themselves, thereby preserving system integrity.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to the field of network architecture and administration, and more particularly to the synchronization of internal times to a reference time source. 
     2. Description of Related Art 
     Large network enterprises spanning multiple time zones or enterprises requiring accurate date stamping on their records and transactions look to a single solution for their enterprise management. Alongside software distribution, distributed monitoring, user administration and security management, time synchronization is important glue for the far-flung organization. Keeping consistent time across the enterprise is not only a good idea but is also a necessity for many applications as illustrated by the following examples: 
     Example 1: An online trading office with day traders working out of their homes needs to ensure that transactions sent for processing are correctly time stamped. The stamps function as an audit for how long a transaction takes to enter, which may be another indication of data manipulation or hacking. 
     Example 2: A casino needs to monitor transactions received from machines to ensure data integrity. 
     Example 3: An IT organization needs to make sure that all machines are working from the same time source, regardless of geography. A breakdown on synchronization indicates either data degradations on lines not under their control, or power faults requiring remediation. 
     A file server in a network timestamps a write to a file with a current time obtained from the server&#39;s clock. A problem is encountered when a server&#39;s clock has a substantially different time than the client&#39;s clock. The problem is that applications running on the client computer can produce errors when, for example, a file is read or accessed with a timestamp ten minutes into the future. 
     Another problem is that a comparison of log entries can be complicated when clients with clocks that differ by minutes are logging to the same host. Attempting to chain together a sequence of events while looking at these logs is often futile due to the inaccurate timestamps. 
     In an effort to solve this problem, the Network Time Protocol (NTP) has been developed. NTP provides mechanisms to synchronize time and coordinate time distribution in a large, diverse internet. The protocol uses a returnable-time design in which a distributed subnet of time servers operating in a self-organizing, heirarchical master/slave configuration synchronizes local clocks within a subnet to national time standards by wire or radio. NTP relies on access to an accurate timekeeping device. The protocol can provide amazingly accurate synchronization between clients and the timekeeper. Across a LAN, accuracies are usually within milliseconds. Within WANs, the accuracy may degrade to tens of milliseconds. However, this is as accurate as most applications need. 
     The basic philosophy of NTP is that there is a universal notion of time to which all machines should synchronize: Universal Coordinated Time (UTC). Rather than trying to synchronize one host with another, each NTP server attempts to synchronize with UTC independently through the best sources of information a particular server can access. 
     The NTP servers are structured in a hierarchy rather than in a flat space or in a cloud. Each server assigns itself a hierarchy level that corresponds with its distance from a UTC source. Servers at the highest hierarchy level (level 1) have direct access to a source for UTC time, such as a radio receiver. Servers at hierarchy level 2 receive time information from level 1 servers, while level 3 servers receive time information from level 2, and so on. The number of levels is limited to 15 in the RFC-1305 standard. 
     An NTP host can operate as both a client and a server. Typically, an NTP client will synchronize with one server. The choice of server depends on a number of factors, including hierarchy level, measured network delay, and claimed precision. However, if three or more servers are available, the client employs a voting algorithm to ensure its server of choice is not providing an obviously incorrect time value. 
     As a client, an NTP host synchronizes with other servers. As a server, it permits other NTP hosts to synchronize with itself. There are two types of association modes between NTP hosts. In a client-server association, the client only receives time information from the server. In the symmetric-active mode, each NTP host acts as a server to the other and the two hosts exchange time information. The typical arrangement for a pool of time servers is for each to use a symmetric-active association with the others in the pool and to use a client association with at least three lower-hierarchy servers. The most robust configuration would have each server use a client association with a different set of three lower-hierarchy servers. 
     User workstations typically will not be slated for the role of NTP server but will be configured to use a client association with each NTP server in the pool. Client-only hosts also can be configured to receive synchronization information via broadcast or multicast packets. In a network of 100 clients and three time servers, for example, there are more than 300 associations and packet exchanges. If the time servers simply broadcast time information, the amount of traffic required to maintain the 100 clients reduces dramatically. Although the broadcast method decreases accuracy, the decrease in accuracy is not significant for most applications. 
     Although a standard for time synchronization has previously been disclosed, there remains a need for a mechanism to implement the standard into various platforms. There is a particular need for software that is cross-platform compatible to ensure compatibility within an enterprise. The software should be capable of being integrated into current network management systems such that the tools of the management system are utilized and the cost of implementing the solution is minimized. Furthermore, the system should provide authentication/certification capabilities based on the time synchronization and should interface the authentication functions with applications in order to assure integrity of operations. 
     SUMMARY OF THE INVENTION 
     The present invention keeps all servers and clients in an enterprise updated to the correct time through a hierarchical network of time servers, with a single time source at the top of the pyramid, and time source clients at the base. 
     Single source time synchronization is assured by starting with an atomic clock receiver connected to a time source computer. This source is accessed by the primary time servers beneath it to obtain accurate time information. Clients access the highest time server to obtain time information. If all of the time servers are unavailable, then the time information is obtained from the next lowest level of servers. 
     Time servers operate as multiple hierarchical layers between the clients and the time source, limiting the load on any single server and providing redundant paths to a valid time source. Time servers unable to verify their time through the primary, alternate, or failover servers invalidate themselves, thereby preserving system integrity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a pictorial representation depicting a data processing system in which the present invention may be implemented in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a block diagram illustrating a data processing system in which the present invention may be implemented. 
     FIG. 3 is a tree diagram of the hierarchy architecture of an enterprise in a preferred embodiment of the invention. 
     FIG. 4 is a flowchart of a preferred embodiment of the invention in the client startup mode of the time client mode. 
     FIG. 5 is a flowchart of a client synchronization mode of the time client mode of a preferred embodiment of the invention. 
     FIG. 6 is a flowchart of the time generator mode of a preferred embodiment of the invention. 
     FIG. 7 is a flowchart of the time server startup mode of a preferred embodiment of the invention. 
     FIG. 8 is a flowchart of the time server synchronization mode of a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures and in particular with reference to FIG. 1, a pictorial representation depicting a data processing system in which the present invention may be implemented in accordance with a preferred embodiment of the present invention. A personal computer  100  is depicted which includes a system unit  110 , a video display terminal  102 , a keyboard  104 , storage devices  108 , which may include floppy drives and other types of permanent and removable storage media, and mouse  106 . Additional input devices may be included with personal computer  100 . Personal computer  100  can be implemented using any suitable computer, such as an IBM Aptiva™ computer, a product of International Business Machines Corporation, located in Armonk, N.Y. Although the depicted representation shows a personal computer, other embodiments of the present invention may be implemented in other types of data processing systems, such as network computers, Web based television set top boxes, Internet appliances, etc. Computer  100  also preferably includes a graphical user interface that may be implemented by means of systems software residing in computer readable media in operation within computer  100 . 
     With reference now to FIG. 2, a block diagram illustrates a data processing system in which the present invention may be implemented. Data processing system  200  is an example of a computer, such as computer  100  in FIG. 1, in which code or instructions implementing the processes of the present invention may be located. Data processing system  200  employs a peripheral component interconnect (PCI) local bus architecture. Although the depicted example employs a PCI bus, other bus architectures such as Accelerated Graphics port (AGP) and Industry Standard Architecture (ISA) may be used. processor  202  and main memory  204  are connected to PCI local bus  206  through PCI bridge  208 . PCI bridge  208  also may include an integrated memory controller and cache memory for processor  202 . Additional connections to PCI local bus  206  may be made through direct component interconnection or through add-in boards. In the depicted example, local area network (LAN) adapter  210 , small computer system interface SCSI host bus adapter  212 , and expansion bus interface  214  are connected to PCI local bus  206  by direct component connection. In contrast, audio adapter  216 , graphics adapter  218 , and audio/video adapter  219  are connected to PCI local bus  206  by add-in boards inserted into expansion slots. Expansion bus interface  214  provides a connection for a keyboard and mouse adapter  220 , modem  222 , and additional memory  224 . SCSI host bus adapter  212  provides a connection for hard disk drive  226 , tape drive  228 , and CD-ROM drive  230 . Typical PCI local bus implementations will support three or four PCI expansion slots or add-in connectors. 
     An operating system runs on processor  202  and is used to coordinate and provide control of various components within data processing system  200  in FIG.  2 . The operating system may be a commercially available operating system such as OS/2, which is available from International Business Machines Corporation. “OS/2” is a trademark of International Business Machines Corporation. An object oriented programming system such as Java may run in conjunction with the operating system and provides calls to the operating system from Java programs or applications executing on data processing system  200 . “Java” is a trademark of Sun Microsystems, Inc. Instructions for the operating system, the object-oriented operating system, and applications or programs are located on storage devices, such as hard disk drive  226 , and may be loaded into main memory  204  for execution by processor  202 . 
     Those of ordinary skill in the art will appreciate that the hardware in FIG. 2 may vary depending on the implementation. Other internal hardware or peripheral devices, such as flash ROM (or equivalent nonvolatile memory) or optical disk drives and the like, may be used in addition to or in place of the hardware depicted in FIG.  2 . Also, the processes of the present invention may be applied to a multiprocessor data processing system. 
     For example, data processing system  200 , if optionally configured as a network computer, may not include SCSI host bus adapter  212 , hard disk drive  226 , tape drive  228 , and CD-ROM  230 , as noted by dotted line  232  in FIG. 2 denoting optional inclusion. In that case, the computer, to be properly called a client computer, must include some type of network communication interface, such as LAN adapter  210 , modem  222 , or the like. As another example, data processing system  200  may be a stand-alone system configured to be bootable without relying on some type of network communication interface, whether or not data processing system  200  comprises some type of network communication interface. As a further example, data processing system  200  may be a Personal Digital Assistant (PDA) device which is configured with ROM and/or flash ROM in order to provide non-volatile memory for storing operating system files and/or user-generated data. 
     The depicted example in FIG.  2  and above-described examples are not meant to imply architectural limitations. For example, data processing system  200  also may be a notebook computer or hand held computer in addition to taking the form of a PDA. Data processing system  200  also may be a kiosk or a Web appliance. 
     Referring now to FIG. 3, a tree diagram of the hierarchy architecture of an enterprise in a preferred embodiment of the invention is shown. At the top of the tree, a time source computer  300  is connected to a radio receiver  305  which receives a signal from a source transmitting an accurate time reference. The time source computer  300  may also be connected to a sister time source computer (not shown) for synchronization when radio receiver  305  is inoperative for some reason. Primary time servers  310 ,  315  are connected to the time source  300  and periodically synchronize their internal time to the time source  300 . Regional servers  320 ,  325  are connected to the primary time servers  310 ,  315 . The regional servers  320 ,  325  are connected such that if primary time server  310  in the left branch of the network is busy or inoperative, the primary time server  315  in the right branch may be accessed even though the regional server is not a direct descendant of the primary time server  315 . The regional servers  320 ,  325  may serve a particular geographic region, a city, or a group of cities, depending on the number of terminal servers and clients contained in the region, city, or group of cities. Within each region, there are a number of terminal servers  330 ,  335  which are connected to the regional server for that region. 
     The terminal servers in FIG. 3 are shown only for one regional server, although all of the other regional servers would also have terminal servers attached as well. The terminal servers are also connected to other regional servers to allow other regional servers to be accessed when the regional server associated with a particular terminal server is unavailable. The lowest level of the hierarchy contains the clients  340  which are connected to a terminal server  330 . The clients  340  may also be connected to other terminal servers  335  in the case that the particular terminal server  330  associated with the clients  340  is busy when a request is made. 
     The servers shown in FIG. 3 may contain different platforms or operating systems from which the server runs. The present invention allows the internal time of each server to be synchronized regardless of the platform of the server. In addition, the method of connecting the network may be any method of connection used by one skilled in the art to form a network. In a preferred embodiment, the network components of the enterprise have TCP/IP stacks installed and available for communication of data. However, the invention is not limited to this particular method of data communication. 
     The present invention comprises an NTP compliant software module that is implemented into the time source  300 , each server, and each client. Preferably, the time source  300  is an IBM 3090 running OS/390, but the time source is not limited to this particular server. A preferred embodiment has three modes of operation: time generator, time server, and time client. In the time generator mode, the time is retrieved from an atomic clock source  305  by the time source  300 . In the time server mode, a server updates its time at a specified interval. This is accomplished by identifying a server that is available for synchronization. In a preferred embodiment, this is done using a “TMR policy region.” This is a functional component of the Tivoli enterprise management software. Servers are identified as network resources, and can be accessed or addressed individually or through a group with which the server is affiliated. The server is then synchronized with the server identified by the TMR policy region. Once synchronization is accomplished, the server may pass on time information to a server or client that requests time synchronization. 
     In the time client mode, the client broadcasts a request for time. The request is then answered by any available time server which is listening to the network subnet in which the client is located. While FIG. 3 shows a particular embodiment with 5 hierarchy levels, the invention is not limited to any particular number of hierarchy levels, or any particular number of time sources, servers, or clients. The invention is scalable to fit any network architecture with any number of components. FIG. 3 is provided for illustrative purposes only and is not intended to limit the application of the invention to other network structures. 
     Referring now to FIG. 4, a flowchart of a preferred embodiment of the invention in the client startup mode of the time client mode is illustrated. Beginning at step  400 , the client attempts to log in to the network. Time synchronization ancestor information is obtained from a configuration file  410  at step  405 . This file is contained on the time server for that client and a copy is retrieved from the server each time a client attempts to synchronize itself. This prevents a hacker from modifying the configuration file. If a “mother” server is available, that is a server which is directly above the client in the hierarchy, then an initial synchronization with the mother is attempted at step  410 . If the synchronization process is successful at step  415 , then the client is logged in to the management system at step  417 . The client then performs the management functions desired and the client is periodically re-synchronized at step  420  as described below in reference to FIG.  5 . 
     However, if initial synchronization with the mother is not successful at step  415 , then an attempt to synchronize the client with an “aunt” server is made at step  425 . An aunt server is a server which is located in the same family and hierarchy level of servers as the mother, but of which the client is not a direct descendant. If the synchronization with the aunt is successful at step  430 , then the configuration file for the client is updated with the last successful ancestor at step  435 , and the client is logged in to the management system at step  415 . 
     If synchronization with the aunt is not successful at step  430 , then an attempt to synchronize the client with a “grandmother” server is made at step  440 . A grandmother server is a server which is located in the same line, two levels removed from the client. If the synchronization with the grandmother is found to be successful at step  430 , then the configuration file is updated with the last successful ancestor at step  435 , and the client is logged in to the management system at step  415 . 
     If synchronization with the grandmother fails as well, then an error is logged to the management system at step  450 , and the client is locked from logging in to the network at step  455 . Then the login process ends at step  460 . However, in other embodiments of the invention, an attempt is made to synchronize the client with other servers in the network tree. Thus, the integrity of the time stamps on various operations is assured because the client is not allowed to log in unless the internal time is properly synchronized. In another embodiment of the invention, the client is allowed to log in to the network despite being unable to synchronize its internal time. However, the management system would place a flag on all operations performed by the client to indicate to the management system that the client has not been authenticated. Thus, the client may be logged on without synchronization and the problem of inaccurate time stamps is avoided by the use of the flags. 
     Once the client is logged into the management system, the internal time of the client is periodically synchronized. Referring now to FIG. 5, a flowchart of a preferred embodiment of the invention in the client synchronization mode of the time client mode is shown. The synchronization process begins at step  500  when a synchronization request is made by the client. The last valid ancestor is retrieved from the configuration file  510  at step  505 . Then a determination is made as to whether the last valid ancestor stored in the configuration file is still valid at step  515  by attempting a time synchronization with the last valid ancestor. If the ancestor returns an authenticated time in response to the attempted time synchronization, then the ancestor is still valid. If the ancestor is invalid at step  515 , the client is logged off the network at step  520  and the startup process is repeated at step  525 . Step  525  represents the startup process shown in FIG.  4 . 
     If the last ancestor is still valid at step  515 , then an attempt is made to synchronize the client with the mother time server at step  530 . Synchronization with the mother is always attempted first in order to avoid overloading the ancestors with time synchronization requests. If the synchronization process with the mother is successful at step  535 , then the time discrepancy that exists between the mother and the client is compared to a predefined range at step  540 . If the discrepancy is out of range, then the discrepancy is logged with the management system at step  545 , and the synchronization process ends at step  550 . The reason that the discrepancy is logged is that the time discrepancy may represent a security breach. Logging the discrepancy does not mean that the synchronization process was unsuccessful, however, but only that there may be a potential security breach. No log is made if the discrepancy is not out of range at step  540 . If synchronization with the mother is not successful at step  535 , then the client startup process is repeated at step  525 . 
     Referring now to FIG. 6, a flowchart of the time generator mode of a preferred embodiment of the invention is illustrated. Starting at step  600 , the time generator mode is entered. The first step in the time generator mode is to pull the last valid sister time sources information from the configuration file  610  located on the time generator server at step  605 . If the configuration of the file is not readable at step  620 , then the process is terminated and an error message may be sent to the operator, Tivoli System Management(TSM), and the Tivoli Enterprise Console (TEC) at step  625 . The Tivoli System Management is the software determined to be appropriate to receive communications regarding the time synchronization issues. The software could be Tivoli Distributed Monitoring, Tivoli Global Enterprise Manger, Tivoli Expert Advisor, Tivoli Service Desk, etc, which are all available from Tivoli Systems, Inc. If the configuration file is readable at step  620 , then the last valid sister time sources in the configuration file are checked to determine if they are still valid at step  615  by attempting a synchronization with the sister time source. 
     If all of the last valid sister time sources pulled from the time generator configuration file  610  are invalid at step  615 , then a new configuration file is created which does not contain sister source assumptions at step  630 . A sister source assumption is a sister time source which is referenced in the configuration file as an active time source. At step  635 , the time generator checks the availability of sister time sources. If no other sister time sources are available, then the process moves to step  640  and the server awaits a time service request from a client or from a sister time source. However, if there are sister time sources available at step  635 , then each sister time source that is active is identified and a notification message is sent to each active sister time source to enable the sister time source to make a time service request to the time generator at step  640 . The configuration file on the time generator is rewritten to reflect active sister time sources at step  645 . The time generator then awaits service requests by a sister time server or a client at step  640 . Note that the term “client” as used here could also refer to a server which is immediately below the time generator in the hierarchy, but which is requesting time synchronization with the time generator. 
     At step  655 , a time service request is received by the time generator. An attempt is made to retrieve the time from the time source at step  660 . The time source in a preferred embodiment is an atomic radio clock or some other customer-specified time input device. If a time value is returned by the time source to the time generator at step  665 , then the time value is sent to the time server that requested the time service using Network Time Protocol (NTP) at step  690 . The time generator then awaits the next time service request at step  650 . 
     If the time source did not return a value at step  665 , then another attempt is made at step  660  to retrieve a value from the time source. This process is repeated three times until a time value is returned. If a value is not returned at step  665  after three retries, then the time generator determines if another sister time source is available at step  670 . If there are no sister time sources available, then an error message is logged to the operator, TSM, and TEC at step  675 . However, if there are sister time sources available at step  670 , then an attempt is made to synchronize with each sister at step  680 . If the synchronization is successful at step  685 , then the time value retrieved from the sister at step  680  is sent to the requesting time server using NTP at step  690 . If the synchronization is not successful at step  685 , then an error message is logged to the operator, TSM, and TEC at step  675 . 
     Referring now to FIG. 7, a flowchart of the startup mode of the time server mode of a preferred embodiment of the invention is illustrated. This mode is entered whenever the time server is logged into the management system. Starting at step  700 , the time server startup mode is entered. The time server retrieves the last valid parent time sources from the time server configuration file  710  at step  705 . The configuration file is checked to see if it is readable at step  720  by attempting a read of the file. If the configuration file is not readable at step  720 , the process is terminated with an error message to the operator, TSM, and TEC at step  725 . 
     If the last valid parent time source retrieved from the time server configuration file at step  705  is currently valid at step  715 , then the client startup mode is entered at step  740 . Attempting a time synchronization is used to determine if the last valid parent time source is currently valid. If the synchronization attempt is successful, then the source is still valid. The client startup mode of step  740  is equivalent to the client startup illustrated by FIG.  4 . Thus, the same software which is used in the client for startup may also be used in the time server for startup. 
     If the last valid parent time source is invalid at step  715 , then a new configuration file is created which does not contain parent source assumptions at step  730 . The client startup procedure of FIG. 4 is performed at step  740 . If the startup is not successful at step  745 , then if the configuration file was modified at step  730  during this startup procedure, the original configuration file is restored at step  760  and the procedure is terminated with an error message to the operator, TSM, and TEC at step  755 . If the configuration file has not been modified, then the procedure simply terminates with an error message because there is no original configuration file to restore. 
     If the startup is successful at step  745 , then the time server searches for sister time servers available for synchronization at step  765 . If there are sister time servers available, then for each sister time source a determination is made as to whether the time source is active. If the time source is active, then a notification message is sent to the sister time source to indicate that the present time server is active at step  770 . Then the configuration file is re-written to reflect the current active sister time sources at step  775 . The time server awaits time service requests from clients or sister time servers at step  780 . If there are no sister time servers available at step  765 , then there is no need to identify the active servers. 
     Referring now to FIG. 8, a flowchart of the time server synchronization mode of a preferred embodiment of the invention is illustrated. At step  800 , a time service request is received by the time server from a client or a sister time server. Upon receiving a time service request, the time server attempts to retrieve a time value from the last valid ancestor at step  805 . If the ancestor returns a value at step  810 , then the time is sent to the client using NTP at step  835 . The time server then awaits the next time service request at step  840 . 
     If the ancestor does not return a value at step  810 , then the time server determines whether time source sisters are available to retrieve the time at step  815 . If there are no sister time sources available, then an error message is logged to the operator, TSM, and TEC at step  820 . However, if there are sister time sources available at step  815 , then an attempt is made to synchronize with the sister time sources at step  825 . If none of the synchronization attempts are successful at step  830 , then an error message is logged to the operator, TSM, and TEC at step  820 . If one of the synchronizations is successful at step  830 , then the time is sent to the client or the sister server that requested the time service using NTP at step  835 . The time server then awaits the next time service request at step  840 . 
     In a preferred embodiment, the management system referenced above is the Tivoli Enterprise Console (TEC) available from Tivoli Systems, Inc. The present invention deals with an automatic authentication/lockout function as described above. However, time synchronization information may also be transmitted to the TEC for further correlation or action depending on what the user wants to do with the information beyond automatic authentication/lockout functions. In the preferred embodiment, the power of management systems is merged with single-source time synchronization using an RFC-1305 compliant protocol. Global accuracy of at least ±10 seconds may be obtained when the invention is implemented on properly powered computers connected to the time source through validated time servers. 
     There is no required or favored platform for any software component of the invention. The software may be implemented on any platform including an IBM 3090 running OS/390, RISC/6000, and Windows NT. The software agent residing on endpoints is cross-compiled for the target operating system. Some operating systems will require additional code when there are known timing problems (such as certain versions of SUN OS and hardware). Time servers in the present invention are cross-platform and may be used to serve heterogeneous networks. Each heterogeneous system (as in a WAN or LAN for a given region or company) has a top-level server for that area, linking back up into a parent server or the primary server. This top-level server may fan out time authentication to other servers in that area. 
     It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in a form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media such as a floppy disc, a hard disk drive, a RAM, and CD-ROMs, and transmission-type media such as digital and analog communications links. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.