Abstract:
A system and method are provided for controlling the computing bandwidth and resources provided to external entities based on subscription levels associated with those external entities. Higher subscription levels provide greater resource allocation. Accounting is accomplished by tracking bandwidth allocated and used over given periods of time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application contains subject matter that may be related to the subject matter in the following U.S. applications filed on Apr. 22, 2005, and assigned to the assignee of the present application: “Method and Apparatus for Consolidating Available Computing Resources on Different Computing Devices” Ser. No. 11/112,368; “Assigning Higher Priority to Transactions Based On Subscription Level” Ser. No. 11/112,947; “Method and Apparatus for Dynamically Isolating Affected Services Under Denial of Service Attack” Ser. No. 11/112,158; “Method and Apparatus for Improving User Experience for Legitimate Traffic of a Service Impacted by Denial of Service Attack” Ser. No. 11/112,629; “Method and Apparatus for Limiting Denial of Service Attack by Limiting Traffic for Hosts” Ser. No. 11/112,328; “Hardware-Based Network Interface Per-Ring Resource Accounting” Ser. No. 11/112,222; “Dynamic Hardware Classification Engine Updating for a Network Interface” Ser. No. 11/112,934; “Network Interface Card Resource Mapping to Virtual Network Interface Cards” Ser. No. 11/112,063; “Network Interface Decryption and Classification Technique” Ser. No. 11/112,436; “Method and Apparatus for Enforcing Resource Utilization of a Container” Ser. No. 11/112,910; “Method and Apparatus for Enforcing Packet Destination Specific Priority Using Threads” Ser. No. 11/112,584; “Method and Apparatus for Processing Network Traffic Associated with Specific Protocols” Ser. No. 11/112,228; and “Method and Apparatus for Enforcing Bandwidth Utilization of a Virtual Serialization Queue” Ser. No. 11/112,322. 
   BACKGROUND 
   Computing systems range from simple systems having one or two central processing units (CPU&#39;s) to complex systems having many nodes, each node having up to forty or more CPU&#39;s. 
   It is common for computing systems to have time periods when computing resources (e.g., processor time, network bandwidth, etc.), which are not used for processing computing jobs, are available. The number and quantity of available resources often depends on the size of the computing system involved. Larger computing systems with more overall capability and available resources than smaller systems may have a large amount of resources available during certain time periods. 
   Further, some computing systems may have unused computing resources at a time when another computing system may have more computing jobs than can possibly be handled by that system. To maximize utilization of resources, operators of computing systems with unused resources offer those resources to external computing systems. External systems using those resources send packets of data to the offering computing system, and those packets are processed in due course with all other packets arriving from other sources, without regard for the source or purpose of the packet. 
   SUMMARY 
   In general, in one aspect, the invention relates to a method for managing computing resources that includes creating multiple virtual network stacks on a first computing system, and assigning one of those virtual network stacks to a first business entity. The assigned virtual network stacks is associated with a priority according to a subscription level of the business entity, and a packet destination. Packets received into the virtual network stack are first classified according to information within the packet, such as connection information, the address of the originating computing system, etc. and are then processed according to the priority associated with the first virtual network stack. 
   In general, in one aspect, other virtual stacks may be associated with other business entities having different subscription levels. Packets arriving at those other virtual network stacks will be processed according to priorities associated with those virtual network stacks, according to the various associated subscription levels. 
   In general, in one aspect, the invention relates to a computer system that includes multiple virtual network stacks, with a first virtual network stack and a second virtual network stack having first and second priorities respectively. The system further includes a network interface configured to receive packets from a first computing system and a second computing system associated with a first business entity and a second business entity respectively, wherein the first business entity and the second business entity are associated with a first subscription level and second subscription level, respectively. 
   In general, in one aspect, a classifier is operatively connected to the network interface and configured to analyze each packet and determine which of the plurality of packets are to be routed to different virtual network stacks within the system. Multiple temporary data structures are individually associated with corresponding ones of virtual network stacks, and are configured to receive packets from the classifier. A computing process is configured to request packets from individual temporary data structures based on first and second priorities respectively associated with respective first and second subscription levels. 
   Other aspects of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a block diagram of a system according to one or more embodiments of the invention. 
       FIG. 2  shows a system in accordance with one or more embodiments of the invention. 
       FIG. 3  shows a virtual serialization queue in accordance with one or more embodiments of the invention. 
       FIG. 4  shows a flowchart of a method according to one or more embodiments of the invention. 
   

   DETAILED DESCRIPTION 
   Exemplary embodiments of the invention will be described with reference to the accompanying drawings. Like items in the drawings are shown with the same reference numbers. 
   In one or more embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention. 
   In this specification, it is intended that the term “coupled” describe hardware and software devices and processes which interact with each other, directly or indirectly. For example, first and second hardware devices that interact with each other through a transmission line between the two devices are directly coupled. Further, first and second devices that have intermediate devices disposed between them, and interact with one another through those intermediate devices, are indirectly coupled. In both situations, the first and second devices are considered coupled. 
   In general, in one or more embodiments of the invention relate to a method and apparatus for creating a pool of computing resources which are provided to others. More specifically, embodiments of the invention relate to a method and apparatus for managing computing resources used by one or more remote computing systems on a per service or per process basis. 
     FIG. 1  shows a block diagram of a system according to one or more embodiments of the invention. Network ( 102 ) includes local computing systems ( 104 ,  106 ,  108 , and  110 ) which periodically have unused resource capacity. Local computing systems ( 104 ,  106 ,  108 , and  110 ) are coupled to each other and to an intermediate system ( 112 ) through transmission lines ( 114 ). Also coupled to the intermediate system ( 112 ) are remote computing systems ( 116 ,  118 ,  120 ,  122 , and  124 ). 
   Persons of ordinary skill in the art having the benefit of this disclosure will readily recognize that systems described as “coupled” above may have intermediate devices disposed between them but still interact and communicate with each other. 
   The terms “local” and “remote” as used above are used only to provide specificity to a particular device or set of devices (such as remote computing systems  116 ,  118 ,  120 ,  122 , and  124 ) being discussed. Thus, those terms are not meant to describe the character of the coupling between or the proximity of the referenced devices. 
   For example, when discussing first and second computing systems coupled together, through a direct connection of a transmission line with no other devices disposed between them, or alternatively coupled together through intermediate devices, the first computing system may be labeled as remote while the second computing system is labeled as local. However, the labels “local” and “remote” could just as easily be applied to the second computing system and the first computing system respectively. 
   Referring again to  FIG. 1 , assume one or more local computing systems ( 104 ,  106 ,  108 , and  110 ) have, at times, unused resource capacity. Further, assume that one or more remote computing systems ( 116 ,  118 ,  120 ,  122 , and  124 ), from time to time, have jobs which cannot be executed due to the lack of resources on those remote computing systems. 
   The invention described herein may be employed to allow one or more remote computing systems ( 116 ,  118 ,  120 ,  122 , and  124 ) to use the unused respective resource capacities of the local computing systems ( 104 ,  106 ,  108 , and  110 ). If desirable, the various local computing systems ( 104 ,  106 ,  108 , and  110 ) may account for such usage, for billing purposes or for other reasons. The intermediate computing system ( 112 ) may also account for the usage of the various resources by various ones of the remote computing systems ( 116 ,  118 ,  120 ,  122 , and  124 ), for billing or other purposes. 
   In one or more embodiments of the invention, owners or operators of the intermediate computing system ( 112 ) make arrangements with the respective owners or operators of the local computing systems ( 104 ,  106 ,  108 , and  110 ) to use excess or unused resource capacity of those systems. The owners or operators of the intermediate system ( 112 ) may make arrangements with the owner or operators of the remote computing systems ( 116 ,  118 ,  120 ,  122 , and  124 ) who need the excess or unused resource capacity from time to time. 
   In addition to providing the availability of unused resource capacity, a local computing system ( 104 ,  106 ,  108 , and  110 ) may wish to control the bandwidth provided to various ones of either the intermediate computing system ( 112 ) or the remote computing systems ( 116 ,  118 ,  120 ,  122 , or  124 ). This bandwidth control is done by examining network traffic, typically in the form of packets, classifying those packets using desirable criteria, and acting on those classified packets according to the level of importance placed on the packet, as determined during the classifying process. 
   Within a local computing system ( 104 ,  106 ,  108 , and  110 ) is a packet processing system used for receiving packets and for processing those packets according to the level of importance, or priority associated with the packets. 
     FIG. 2  shows a system in accordance with one or more embodiments of the invention. Computing system ( 200 ) includes a host ( 202 ) operatively connected to a network interface card (NIC) ( 204 ). The NIC ( 204 ) provides an interface between the host ( 202 ) and a network (not shown) (e.g., a local area network, a wide area network, a wireless network, etc.). More specifically, the NIC ( 204 ) includes a network interface (i.e., the hardware used to interface with the network). 
   Packets received at the network interface are forwarded to other components on the NIC ( 204 ) for processing. In one or more embodiments of the invention, the NIC ( 204 ) includes a classifier ( 206 ) and one or more receive rings (e.g.,  208 A,  208 B,  208 C). In one or more embodiments of the invention, the receive rings ( 208 A,  208 B,  208 C) correspond to portions of memory within the NIC ( 204 ) used to temporarily store the received packets. Further, in one or more embodiments of the invention, a ring element of the receive rings ( 208 A,  208 B,  208 C) may point to host memory. In one or more embodiments of the invention, the classifier ( 206 ) is configured to analyze the incoming network traffic, typically in the form of packets, received from the network (not shown), in order to ultimately determine which virtual network stack should receive each packet. The NIC ( 204 ) is coupled through a device driver ( 210 ) to virtual network stacks, such as virtual network stacks  212 A,  212 B, and  212 C. 
   This determination is made based on a number of factors, some of which may be system specific. In one or more embodiments of the invention, the determination as to which virtual network stack (e.g.,  212 A,  212 B, or  212 C) should receive a given packet is based on the connection used to transmit the packet into the system ( 200 ). 
   In one or more embodiments of the invention, analyzing packets includes analyzing information within the packets or associated with the packets (e.g., connection information, connection attributes, etc.) to make the determination. 
   The classifier ( 206 ) may be implemented entirely in hardware (i.e., the classifier ( 206 ) may be a separate microprocessor embedded on the NIC ( 204 )). Alternatively, the classifier ( 206 ) may be implemented in software stored in memory (e.g., firmware, etc.) on the NIC ( 204 ) or within the host ( 202 ) and executed by a microprocessor on the NIC ( 204 ) or within the host ( 202 ). 
   Once the classifier ( 206 ) has analyzed and classified a given packet, that packet is sent to the appropriate receive ring (e.g., one of receive rings  208 A,  208 B,  208 C,), which hold packets awaiting processing that share at least one common characteristic. 
   In one or more embodiments of the invention, the device driver ( 210 ) provides an interface between the receive rings ( 208 A,  208 B,  208 C) and the host ( 202 ). The virtual network stacks ( 212 A,  212 B,  212 C) provide an abstraction layer between the NIC ( 204 ) and the various packet destination(s) ( 214 ) (e.g., container(s) and/or service(s)) executing on the host ( 202 ). 
   In one or more embodiments of the invention, a virtual network stack (e.g.,  212 A,  212 B,  212 C) includes a virtual network interface card (VNIC) ( 216 A,  216 B,  216 C), a virtual protocol stack (e.g.,  218 A,  218 B,  218 C) and a virtual serialization queue (e.g.,  220 A,  220 B,  220 C). 
   More specifically, each VNIC ( 216 A,  216 B,  216 C) operates like a physical NIC ( 204 ). For example, in one or more embodiments of the invention, each VNIC ( 216 A,  216 B,  216 C) is associated with an Internet Protocol (IP) address and one or more ports, and is configured to handle one or more protocol types. Thus, while the host ( 202 ) may be operatively connected to a single NIC ( 204 ), packet destination(s) ( 214 ) executing on the host ( 202 ) operate as if the host ( 202 ) includes multiple NICs. In one or more embodiments of the invention, the receive rings ( 208 A,  208 B,  208 C) and queues (i.e., buffers) associated with the virtual NIC ( 216 A,  216 B,  216 C) may be generally referred to as temporary data structures. 
   Each of the VNICs ( 216 A,  216 B,  216 C) is operatively connected to a corresponding virtual protocol stack ( 218 A,  218 B,  218 C). In one or more embodiments of the invention, each virtual protocol stack ( 218 A,  218 B, and  218 C) includes functionality to process packets in accordance with various protocols used to send and receive packets (e.g., Transmission Communication Protocol (TCP), Universal Datagram Protocol (UDP), IP, etc.). Higher level protocols supported by other network layers include Hypertext Transport Protocol (HTTP) and Secure Hypertext Transport Protocol (HTTPS). Further, each virtual protocol stack ( 218 A,  218 B,  218 C) also includes functionality, as needed, to perform additional processing on the incoming and outgoing packets. This additional processing may include, but is not limited to, cryptographic processing, firewall routing, etc. 
   In one or more embodiments of the invention, each virtual protocol stack ( 218 A,  218 B,  218 C) includes network layer and transport layer functionality. In one or more embodiments of the present invention, network layer functionality corresponds to functionality to manage packet addressing and delivery on a network (e.g., functionality to support IP, Address Resolution Protocol (ARP), Internet Control Message Protocol, etc.). In one or more embodiments of the invention, transport layer functionality corresponds to functionality to manage the transfer of packets on the network and functionality to ensure that received packets are identical to transmitted packets (e.g., functionality to support TCP, User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc.). 
   In one or more embodiments of the invention, each virtual protocol stack ( 218 A,  218 B,  218 C) is associated with a particular virtual serialization queue ( 220 A,  220 B,  220 C). 
   In one or more embodiments of the invention, each virtual serialization queue ( 220 A,  220 B,  220 C) includes a data structure having at least two queues, an inbound queue and an outbound queue. Each of the queues within the virtual serialization queues ( 220 A,  220 B,  220 C) are typically implemented as first-in first-out (FIFO) queues. Further, in one or more embodiments of the invention, each virtual serialization queue ( 220 A,  220 B,  220 C) is configured to send and receive packets from an associated VNIC ( 216 A,  216 B,  216 C) via an associated virtual protocol stack ( 218 A,  218 B,  218 C). In addition, each virtual serialization queue ( 220 A,  220 B,  220 C) is configured to send packets to and receive from one or more associated packet destination(s) ( 214 ) (e.g., containers and/or services). 
   The host ( 202 ) may include one or more CPUs ( 222 A,  222 B). Further, each virtual serialization queue ( 220 A,  220 B,  220 C) is bound to one of the CPUs ( 222 A,  222 B). Thus, more than one virtual serialization queue ( 220 A,  220 B,  220 C) may be bound to a given CPU. Further, in some instances, more than one CPU may service a given virtual serialization queue (e.g.,  220 A,  220 B,  220 C). 
   As discussed above, the host ( 202 ) includes one or more packet destinations ( 214 ) (e.g., containers and/or services). In one or more embodiments of the invention, the packet destinations ( 214 ) (e.g., containers and/or services) correspond to a process or group of processes executing on the host that sends and receives network traffic. Examples of packet destinations ( 214 ) include, but are not limited to, containers, zones, web server, etc. 
     FIG. 3  shows a virtual serialization queue in accordance with one or more embodiments of the invention. In one or more embodiments of the present invention, a virtual serialization queue (for example, virtual serialization queue  220 A of  FIG. 2 ) includes a packet scheduler ( 302 ) and one or more sub-virtual serialization queues ( 304 A,  304 B,  304 C). 
   In one or more embodiments of the invention, each sub-virtual serialization queue ( 304 A,  304 B,  304 C) may be configured to queue specific types of packets. For example, the sub-virtual serialization queues ( 304 A,  304 B,  304 C) may be configured to queue received packets based on the protocol (e.g., IP Security Protocol (IPsec), TCP, IP, UDP, etc.) used to send the packet. 
   Persons having ordinary skill in the art having the benefit of this disclosure will appreciate that each sub-virtual serialization queue ( 304 A,  304 B,  304 C) may be configured to queue any distinct subset of packets. In one or more embodiments of the invention, each sub-virtual serialization queue ( 304 A,  304 B,  304 C), is bound to the same CPU (i.e.  222 A of  FIG. 2 ) and associated with the same virtual network stack (i.e.,  212 A of  FIG. 2 ) as the corresponding virtual serialization queue ( 220 A). 
   Further, if the virtual serialization queue ( 220 A) includes one or more sub-virtual serialization queues ( 304 A,  304 B,  304 C), the associated virtual network stack (i.e.,  212 A of  FIG. 2 ) is bound to a corresponding number of receive rings (receive rings not shown). Thus, when the virtual serialization queue ( 220 A) receives packets from one or more receive rings, the packets are routed to the appropriate sub-virtual serialization queue ( 304 A,  304 B,  304 C) based on which receive ring previously held those packets. In one or more embodiments of the invention, each of the sub-virtual serialization queues ( 304 A,  304 B,  304 C) includes a pair of FIFO queues, namely an inbound queue and an outbound queue. 
   Persons of ordinary skill in the art having the benefit of this disclosure will appreciate that a virtual serialization queue ( 220 A) does not necessarily include any sub-virtual serialization queues ( 304 A,  304 B,  304 C), in which case the virtual serialization queue ( 220 A) need only include a pair of queues, one for inbound packets and one for outbound packets. 
   In one or more embodiments of the invention, the packet scheduler ( 302 ) is configured to process the packets stored in each of the associated sub-virtual serialization queues ( 304 A,  304 B,  304 C). More specifically, the packet scheduler ( 302 ) schedules when packets queued in the various sub-virtual serialization queues ( 304 A,  304 B,  304 C) are to be processed (i.e., the order of processing of those packets, etc.). 
   In one or more embodiments of the invention, the packet scheduler ( 302 ) includes functionality to support fair-share scheduling of packets queued on the sub-virtual serialization queues ( 304 A,  304 B,  304 C). In one or more embodiments of the invention, the packet scheduler ( 302 ) includes functionality to support fair-share scheduling of packets queued on the sub-virtual serialization queues ( 304 A,  304 B,  304 C). Further, the packet scheduler ( 302 ) may be configured to schedule packet processing based on individual priorities associated with ones of the sub-virtual serialization queues ( 304 A,  304 B,  304 C). 
   Combining the teachings of  FIGS. 1 through 3 , a distributed system may be obtained by the owners or operators of computing systems establishing virtual network stacks (such as VNS&#39;s  212 A,  212 B, and  212 C) on their respective computing systems, and offering services associated with those virtual network stacks to owners and operators of remote systems (such as remote systems  116 ,  118 ,  120 ,  122 , and  124  of  FIG. 1 ). Although overall control of the processes and priorities on local systems ( 104 ,  106 ,  108 , and  110 ) resides with the owners and operators of those systems, control of the application deployment within a given packet destination and the computing environment within the virtual network stack is provided to the user of the associated resources. 
     FIG. 4  is a flowchart showing a method according to one or more embodiments of the invention. Referring to  FIG. 4 , the availability of resources in the system (such as system  100  of  FIG. 1 ) are determined at  402 . Such a determination is routinely completed by a system scheduler that manages the computing jobs being processed and executed by the system at any given time. Resources may include, but are not limited to storage space, CPU bandwidth, and memory. 
   In one or more embodiments of the invention, the availability of resources may be referred to as a percentage of the available (i.e., unused) resources or a total amount of available (i.e., unused) resources. For example, the availability of resources may be 80% of CPU capacity for a particular computer system. 
   At  404 , a packet is received over a network from a second computing system. Persons of ordinary skill in the art having the benefit of this disclosure are readily aware that packets may be received by a computing system using a wide variety of protocols, some of which use connections (such as TCP/IP), others of which are connectionless (such as UDP). In one or more embodiments of the invention, the second computing system sends the packet over the network following a determination that available resources exist on the first computing system to process the packets. Such a determination may be made through receipt of information from the first computing system (such as computing system  104 ) indicating that availability. Alternatively, a computing system (such as remote computing system  116 ) needing to use resources on an external system (such as local computing system  104 ) may issue a request to use resources, and a computing system having those resources to offer may affirmatively respond. 
   At  406 , attributes of the connection associated with the received packet are determined. In one or more embodiments of the invention, determination of the attributes of the received packet is accomplished by examining the packet itself (e.g., the packet header, the payload, etc.). In one or more embodiments of the invention, such attributes may include the internet protocol (IP) address of the sending computing system, the port over which the packet was received, the protocol used to transmit the packet, etc. Other attributes will be known to persons of ordinary skill in the art having the benefit of this disclosure. It is expected that one or more attributes associated with the received packet will be unique to one or more virtual serialization queues (such as  220 A,  220 B,  220 C of  FIG. 2 ) in the system. 
   At  408 , the received packet is classified according to the one or more attributes determined at  406 . In system ( 200 ) of  FIG. 2 , the different virtual serialization queues (such as  218 A,  218 B, and  218 C of  FIG. 2 ) within the virtual network stack may be associated with different system priorities. Because received packets having similar characteristics are routed to the same virtual serialization queue (such as virtual serialization queue  218 A of  FIG. 2 ), the system may operate on each of the packets according to the level of importance associated with each different virtual serialization queue containing those similar packets. In one or more embodiments of the invention, the classified packet is then placed into a temporary data structure associated with the appropriate virtual network stack (e.g., the virtual network stack associated with the IP address of a particular secondary computer). Further, in one or more embodiments of the invention, the appropriate virtual network stack is given the lowest priority in terms of network bandwidth and CPU resources consumed. 
   At  410 , the received packet is requested by an executing process and routed to the proper virtual serialization queue (such as  220 B of  FIG. 2 ) based on the classification of the packet performed at  408 . Specifically, in one or more embodiments of the invention, the received packet is pulled from the temporary data structure and routed to the virtual network stack. In particular the packet is routed from the temporary data structure to a virtual network interface card, responsive to the classifying of the packet performed at  408 . Next, the packet is routed to a virtual protocol stack associated with the virtual network interface card, and then routed to a virtual serialization queue associated with the virtual protocol stack. 
   In one or more embodiments of the invention, because the virtual network stack associated with the received packet is given the lowest priority, the received packet may remain on the temporary data structure until the virtual network stack(s) (and particularly the virtual serialization queue(s)) with a higher priority have been serviced. 
   At  412 , the packets in a given virtual serialization queue (such as  220 B of  FIG. 2 ) are processed once the priority assigned to the virtual serialization queue (such as  220 B of  FIG. 2 ) is sufficiently high as compared to the availability of resources and the priority of the virtual serialization queue(s) associated with the first computing system. 
   At  414 , an accounting is made of the resources utilized in routing and/or processing the packet. Using the present invention, a packet transmitted by a remote computing system (such as remote computing system  116 ) may pass through intermediate computing system ( 112 ) before being finally processed by a local computing system such as local computing system ( 104 )). Having established a virtual stack within computing system ( 104 ) through which packets must pass in order to be acted upon by a corresponding CPU, computing system ( 104 ) is able to track and have a detailed understanding of how much bandwidth and other resources were used by a given connection. Thus, computing system ( 104 ) is able to account for all resources used by system ( 112 ), and the connections associated with the expenditures of those resources. Because each packet passes through a virtual network stack prior to be further acted on at a packet destination, the virtual network stack may account for bandwidth consumed by counting the number of packets passing through the virtual network stack over a given period of time. 
   Correspondingly, because intermediate computing system ( 112 ) knows which incoming connections (from remote computing systems  116 ,  118 ,  120 ,  122  and  124 ) are associated with the use of resources at local computing systems ( 104 ,  106 ,  108 , and  110 ), invoices may be prepared, if desired, so that the owners and operators of remote computing systems ( 116 ,  118 ,  120 ,  122  and  124 ) may pay according to their respective resource usage. 
   Persons of ordinary skill in the art having the benefit of this disclosure will understand the ability to vary the amount and type of resources allocated to a given network stack, and therefore to a user, based on the load on the computing system. In one or more embodiments of the invention, hard limits and soft limits are established to ensure that some work is done for each user (and thus each virtual network stack) even when the computing system has a very high load on it. These limits also allocate additional resources to the resource user at times when the number of other computing jobs has diminished. 
   Persons of ordinary skill in the art having the benefit of this disclosure will appreciate that software instructions to perform embodiments of the invention may be stored on a computer readable medium such as a flash memory, a compact disc (CD), DVD, a diskette, a tape, a file, or any other computer readable storage device. 
   While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.