Request routing

[0001] Generally described, computing devices and communication networks can be utilized to exchange information. In a common application, a computing device can request content from another computing device via the communication network. For example, a user at a personal computing device can utilize a software browser application to request a Web page from a server computing device via the Internet. In such embodiments, the user computing device can be referred to as a client computing device and the server computing device can be referred to as a content provider.

[0002] Content providers are generally motivated to provide requested content to client computing devices often with consideration of efficient transmission of the requested content to the client computing device and/or consideration of a cost associated with the transmission of the content. For larger scale implementations, a content provider may receive content requests from a high volume of client computing devices which can place a strain on the content provider's computing resources. Additionally, the content requested by the client computing devices may have a number of components, which can further place additional strain on the content provider's computing resources.

[0003] With reference to an illustrative example, a requested Web page, or original content, may be associated with a number of additional resources, such as images or videos, that are to be displayed with the Web page. In one specific embodiment, the additional resources of the Web page are identified by a number of embedded resource identifiers, such as uniform resource locators (“URLs”). In turn, software on the client computing devices typically processes embedded resource identifiers to generate requests for the content. Often, the resource identifiers associated with the embedded resources reference a computing device associated with the content provider such that the client computing device would transmit the request for the additional resources to the referenced content provider computing device. Accordingly, in order to satisfy a content request, the content provider would provide client computing devices data associated with the Web page as well as the data associated with the embedded resources.

[0004] Some content providers attempt to facilitate the delivery of requested content, such as Web pages and/or resources identified in Web pages, through the utilization of a content delivery network (“CDN”) service provider. A CDN server provider typically maintains a number of computing devices in a communication network that can maintain content from various content providers. In turn, content providers can instruct, or otherwise suggest to, client computing devices to request some, or all, of the content provider's content from the CDN service provider's computing devices.

[0005] With reference to previous illustrative example, the content provider can leverage a CDN service provider with the modification or substitution of resource identifiers associated with the embedded resources. Specifically, the resource identifiers can reference a computing device associated with the CDN service provider such that the client computing device would transmit the request for the additional resources to the referenced CDN service provider computing device. Typically, the content provider facilitates the utilization of a CDN provider by including CDN-provider specific resources identifiers in requested content (e.g., Web pages). This approach generally corresponds to an “offline” process implemented by the content provider in advance of receipt of a request for the original content from the client computing devices. Accordingly, modifications to resource identifiers, such as to provide alternative resources identifiers for the same CDN service provider, to provide additional information utilized by CDN service providers in processing the request for content and/or to identify alternative CDN service providers, can become inefficient as they typically require implementation of the offline process the content provider.

[0015] Generally described, the present disclosure is directed to routing of DNS query from a client computing device corresponding to content requests by a network resource, such as content delivery network (“CDN”) service providers. Specifically, aspects of the disclosure will be described with regard to the routing of a client computing device DNS query within a content delivery network service provider domain utilizing one or more alternative resource identifiers. Although various aspects of the disclosure will be described with regard to illustrative examples and embodiments, one skilled in the art will appreciate that the disclosed embodiments and examples should not be construed as limiting.

[0016] FIG. 1 is a block diagram illustrative of content delivery environment 100 for the management and processing of content requests. As illustrated in FIG. 1, the content delivery environment 100 includes a number of client computing devices 102 (generally referred to as clients) for requesting content from a content provider and/or a CDN service provider. In an illustrative embodiment, the client computing devices 102 can corresponds to a wide variety of computing devices including personal computing devices, laptop computing devices, hand-held computing devices, terminal computing devices, mobile devices, wireless devices, various electronic devices and appliances and the like. In an illustrative embodiment, the client computing devices 102 include necessary hardware and software components for establishing communications over a communication network 108, such as a wide area network or local area network. For example, the client computing devices 102 may be equipped with networking equipment and browser software applications that facilitate communications via the Internet or an intranet.

[0017] Although not illustrated in FIG. 1, each client computing device 102 utilizes some type of local DNS resolver component, such as a DNS Name server, that generates the DNS queries attributed to the client computing device. In one embodiment, the local DNS resolver component may be provide by an enterprise network to which the client computing device 102 belongs. In another embodiment, the local DNS resolver component may be provided by an Internet Service Provider (ISP) that provides the communication network connection to the client computing device 102.

[0018] The content delivery environment 100 can also include a content provider 104 in communication with the one or more client computing devices 102 via the communication network 108. The content provider 104 illustrated in FIG. 1 corresponds to a logical association of one or more computing devices associated with a content provider. Specifically, the content provider 104 can include a web server component 110 corresponding to one or more server computing devices for obtaining and processing requests for content (such as Web pages) from the client computing devices 102. The content provider 104 can further include an origin server component 112 and associated storage component 114 corresponding to one or more computing devices for obtaining and processing requests for network resources from the CDN service provider. One skilled in the relevant art will appreciate that the content provider 104 can be associated with various additional computing resources, such additional computing devices for administration of content and resources, DNS name servers, and the like. For example, although not illustrated in FIG. 1, the content provider 104 can be associated with one or more DNS name server components that would be authoritative to resolve client computing device DNS queries corresponding to a domain of the content provider.

[0019] With continued reference to FIG. 1, the content delivery environment 100 can further include a CDN service provider 106 in communication with the one or more client computing devices 102 and the content providers 104 via the communication network 108. The CDN service provider 106 illustrated in FIG. 1 corresponds to a logical association of one or more computing devices associated with a CDN service provider. Specifically, the CDN service provider 106 can include a number of Point of Presence (“POP”) locations 116, 122, 128 that correspond to nodes on the communication network 108. Each POP 116, 122, 128 includes a DNS component 118, 124, 130 made up of a number of DNS server computing devices for resolving DNS queries from the client computers 102. Each POP 116, 122, 128 also includes a resource cache component 120, 126, 132 made up of a number of cache server computing devices for storing resources from content providers and transmitting various requested resources to various client computers. The DNS components 118, 124 and 130 and the resource cache components 120, 126 132 may further include additional software and/or hardware components that facilitate communications including, but not limited, load balancing or load sharing software/hardware components.

[0020] In an illustrative embodiment, the DNS component 118, 124, 130 and resource cache component 120, 126, 132 are considered to be logically grouped, regardless of whether the components, or portions of the components, are physically separate. Additionally, although the POPs 116, 122, 128 are illustrated in FIG. 1 as logically associated with the CDN Provider 106, the POPs will be geographically distributed throughout the communication network 108 in a manner to best serve various demographics of client computing devices 102. Additionally, one skilled in the relevant art will appreciate that the CDN service provider 106 can be associated with various additional computing resources, such additional computing devices for administration of content and resources, and the like.

[0021] With reference now to FIGS. 2-7, the interaction between various components of the content delivery environment 100 of FIG. 1 will be illustrated. For purposes of the example, however, the illustration has been simplified such that many of the components utilized to facilitate communications are not shown. One skilled in the relevant art will appreciate that such components can be utilized and that additional interactions would accordingly occur without departing from the spirit and scope of the present disclosure.

[0022] With reference to FIG. 2, an illustrative interaction for registration of a content provider 104 with the CDN service provider 106 will be described. As illustrated in FIG. 2, the CDN content registration process begins with registration of the content provider 104 with the CDN service provider 106. In an illustrative embodiment, the content provider 104 utilizes a registration application program interface (“API”) to register with the CDN service provider 106 such that the CDN service provider 106 can provide content on behalf of the content provider 104. The registration API includes the identification of the origin server 112 of the content provider 104 that will provide requested resources to the CDN service provider 106.

[0023] One skilled in the relevant art will appreciate that upon identification of appropriate origin servers 112, the content provider 104 can begin to direct requests for content from client computing devices 102 to the CDN service provider 106. Specifically, in accordance with DNS routing principles, a client computing device request corresponding to a resource identifier would eventually be directed toward a POP 116, 122, 128 associated with the CDN service provider 106. In the event that the resource cache component 120, 126, 132 of a selected POP does not have a copy of a resource requested by a client computing device 102, the resource cache component will request the resource from the origin server 112 previously registered by the content provider 104.

[0024] With continued reference to FIG. 2, upon receiving the registration API, the CDN service provider 106 obtains and processes the registration information. In an illustrative embodiment, the CDN service provider 106 can then generate additional information that will be used by the client computing devices 102 as part of the content requests. The additional information can include, without limitation, client identifiers, such as client identification codes, content provider identifiers, such as content provider identification codes, executable code for processing resource identifiers, such as script-based instructions, the like. One skilled in the relevant art will appreciate that various types of additional information may be generated by the CDN service provider 106 and that the additional information may be embodied in any one of a variety of formats.

[0025] The CDN service provider 106 returns an identification of applicable domains for the CDN service provider (unless it has been previously provided) and any additional information to the content provider 104. In turn, the content provider 104 can then process the stored content with content provider specific information. In one example, as illustrated in FIG. 2, the content provider 104 translates resource identifiers originally directed toward a domain of the origin server 112 to a domain corresponding to the CDN service provider. The translated URLs are embedded into requested content in a manner such that DNS queries for the translated URLs will resolve to a DNS sever corresponding to the CDN service provider 106 and not a DNS server corresponding to the content provider 104. Although the translation process is illustrated in FIG. 2, in some embodiments, the translation process may be omitted in a manner described in greater detail below.

[0026] Generally, the identification of the resources originally directed to the content provider 104 will be in the form of a resource identifier that can be processed by the client computing device 102, such as through a browser software application. In an illustrative embodiment, the resource identifiers can be in the form of a uniform resource locator (“URL”). Because the resource identifiers are included in the requested content directed to the content provided, the resource identifiers can be referred to generally as the “content provider URL.” For purposes of an illustrative example, the content provider URL can identify a domain of the content provider 104 (e.g., contentprovider.com), a name of the resource to be requested (e.g., “resource.xxx”) and a path where the resource will be found (e.g., “path”). In this illustrative example, the content provider URL has the form of:

• • [0027] http://www.contentprovider.com/path/resource.xxx

[0028] During an illustrative translation process, the content provider URL is modified such that requests for the resources associated with the translated URLs resolve to a POP associated with the CDN service provider 106. In one embodiment, the translated URL identifies the domain of the CDN service provider 106 (e.g., “cdnprovider.com”), the same name of the resource to be requested (e.g., “resource.xxx”) and the same path where the resource will be found (e.g., “path”). Additionally, the translated URL can include additional processing information (e.g., “additional information”). The translated URL would have the form of:

• • [0029] http://additional_information.cdnprovider.com/path/resources.xxx

[0030] In another embodiment, the information associated with the CDN service provider 106 is included the modified URL, such as through prepending or other techniques, such that the translated URL can maintain all of the information associated with the original URL. In this embodiment, the translated URL would have the form of:

• • [0031] http://additional_information.cdnprovider.com/www.contentprovider.com/path /resource.xxx

[0032] With reference now to FIG. 3, after completion of the registration and translation processes illustrated in FIG. 2, a client computing device 102 subsequently generates a content request that is received and processed by the content provider 104, such as through the Web server 110. In accordance with an illustrative embodiment, the request for content can be in accordance with common network protocols, such as the hypertext transfer protocol (“HTTP”). Upon receipt of the content request, the content provider 104 identifies the appropriate responsive content. In an illustrative embodiment, the requested content can correspond to a Web page that is displayed on the client computing device 102 via the processing of information, such as hypertext markup language (“HTML”), extensible markup language (“XML”), and the like. The requested content can also include a number of embedded resource identifiers, described above, that corresponds to resource objects that should be obtained by the client computing device 102 as part of the processing of the requested content. The embedded resource identifiers can be generally referred to as original resource identifiers or original URLs.

[0033] Upon receipt of the requested content, the client computing device 102, such as through a browser software application, begins processing any of the markup code included in the content and attempts to acquire the resources identified by the embedded resource identifiers. Accordingly, the first step in acquiring the content correspond to the issuance, by the client computing device 102 (through its local DNS resolver), a DNS query for the Original URL resource identifier that results in the identification of a DNS server authoritative to the “.” and the “com” portions of the translated URL. After resolving the “.” and “com” portions of the embedded URL, the client computing device 102 then issues a DNS query for the resource URL that results in the identification of a DNS server authoritative to the “.cdnprovider” portion of the embedded URL. The issuance of DNS queries corresponding to the “.” and the “com” portions of a URL are well known and have not been illustrated.

[0034] With reference now to FIG. 4, in an illustrative embodiment, the successful resolution of the “cdnprovider” portion of the original URL identifies a network address, such as an IP address, of a DNS server associated with the CDN service provider 106. In one embodiment, the IP address is a specific network address unique to a DNS server component of a POP. In another embodiment, the IP address can be shared by one or more POPs. In this embodiment, a further DNS query to the shared IP address utilizes a one-to-many network routing schema, such as anycast, such a specific POP will receive the request as a function of network topology. For example, in an anycast implementation, a DNS query issued by a client computing device 102 to a shared IP address will arrive at a DNS server component logically having the shortest network topology distance, often referred to as network hops, from the client computing device. The network topology distance does not necessarily correspond to geographic distance. However, in some embodiments, the network topology distance can be inferred to be the shortest network distance between a client computing device 102 and a POP.

[0035] With continued reference to FIG. 4, in either of the above identified embodiments (or any other embodiment), a specific DNS server in the DNS component 118 of a POP 116 receives the DNS query corresponding to the original URL from the client computing device 102. Once one of the DNS servers in the DNS component 118 receives the request, the specific DNS server attempts to resolve the request. In an illustrative embodiment, a specific DNS server can resolve the DNS query by identifying an IP address of a cache server component that will process the request for the requested resource. As described above, a selected resource cache component can process the request by either providing the requested resource if it is available or attempt to obtain the requested resource from another source, such as a peer cache server computing device or the origin server 112 of the content provider 104.

[0036] As an alternative to selecting a cache server component, the CDN service provider 106 can maintain sets of various alternative resource identifiers. The alternative resource identifiers can be provided by the CDN service provider 106 to the client computing device 102 such that a subsequent DNS query on the alternative resource identifier will resolve to a different DNS server component within the CDN service provider's network. In an illustrative embodiment, the alternative resource identifiers are in the form of one or more canonical name (“CNAME”) records. In one embodiment, each CNAME record identifies a domain of the CDN service provider 106 (e.g., “cdnprovider.com” or “cdnprovider-1.com”). As will be explained in greater detail below, the domain in the CNAME does not need to be the same domain found in original URL or in a previous CNAME record. Additionally, each CNAME record includes additional information, such as request routing information, (e.g., “request routing information”). An illustrative CNAME record can have the form of:

• • [0037] http://request_routing_information.cdnprovider.com

[0038] In an illustrative embodiment, the CNAME records are generated and provided by the DNS servers to direct a more appropriate DNS server of the CDN service provider 106. As used in accordance with the present disclosure, appropriateness can be defined in any manner by the CDN service provider 106 for a variety of purposes. In one example, the CDN service provider 106 can attempt to direct a DNS query to DNS severs according to geographic criteria. The geographic criteria can correspond to geographic-based regional service plans contracted between the CDN service-provider 106 and the content provider 104 in which various CDN service provider 106 POPs are grouped into geographic regions. Accordingly, a client computing device 102 DNS query received in a region not corresponding to the content provider's regional plan may be better processed by a DNS server in region corresponding to the content provider's regional plan. In this example, the DNS server component 118 may also obtain geographic information from the client directly (such as information provided by the client computing device or ISP) or indirectly (such as inferred through a client computing device's IP address).

[0039] In another example, the CDN service provider 106 can attempt to direct a DNS query to DNS severs according to service level criteria. The service level criteria can correspond to service or performance metrics contracted between the CDN service provider 106 and the content provider 104. Examples of performance metrics can include latencies of data transmission between the CDN service provider POPs and the client computing devices 102, total data provided on behalf of the content provider 104 by the CDN service provider POPs, error rates for data transmissions, and the like.

[0040] In still a further example, the CDN service provider 106 can attempt to direct a DNS query to DNS severs according to network performance criteria. The network performance criteria can correspond to measurements of network performance for transmitting data from the CDN service provider POPs to the client computing device 102. Examples of network performance metrics can include network data transfer latencies (measured by the client computing device or the CDN service provider 106, network data error rates, and the like.

[0041] In accordance with an illustrative embodiment, the DNS server maintains a data store that defines CNAME records for various original URLs. If a DNS query corresponding to a particular original URL matches an entry in the data store, the DNS server component 118 returns a CNAME record as defined in the data store. In an illustrative embodiment, the data store can include multiple CNAME records corresponding to a particular original URL. The multiple CNAME records would define a set of potential candidates that can be returned to the client computing device. In such an embodiment, the DNS server component 118, either directly or via a network-based service, can implement additional logic in selecting an appropriate CNAME from a set of possible of CNAMEs. In an illustrative embodiment, each DNS server component 118, 124, 130 maintains the same data stores that define CNAME records, which can be managed centrally by the CDN service provider 106. Alternatively, each DNS server component 118, 124, 130 can have a POP specific data stores that define CNAME records, which can be managed centrally by the CDN service provider 106 or locally at the POP 116, 122, 128.

[0042] The returned CNAME can also include request routing information that is different from or in addition to the information provided in URL/CNAME of the current DNS query. For example, if the CNAME selection is based on regional plan, a specific regional plan can be identified in the “request_routing_information” portion of the specific CNAME record. A similar approach could be taken to identify service level plans and file management by including a specific identifier in the “request_routing_information” portion of the CNAME record. In another embodiment, request routing information can be found in the identification of a CDN service provider 106 domain different from the domain found in the current URL/CNAME. For example, if the CNAME is based on regional plan, a specific regional plan domain (e.g., “cdnprovder-region1.com”) could be used in the domain name portion of the specific CNAME record. Any additional request routing information can be prepended to the existing request routing information in the current URL/CNAME such that the previous request routing information would not be lost (e.g., http://serviceplan.regionalplan.cdnprovider.com). One skilled in the relevant art will appreciate that additional or alternative techniques and/or combination of techniques may be used to include the additional request routing information in the CNAME record that is selected by the DNS server component 118.

[0043] With continued reference to FIG. 4, one skilled in the relevant art DNS server component 118 may select (or otherwise obtain) a CNAME record that is intended resolve to a more appropriate DNS server of the CDN service provider 106. In may be possible, however, that the same DNS server would also be authoritative for the subsequent DNS query for the CNAME to be provided to the client computing device. For example, a specific DNS server may be authoritative for a both a specific regional plan and a service level plan. Thus, returning a CNAME would still result in the DNS query arriving at the same DNS query (may due in part to the client computing device's geography). In such an embodiment, the DNS server, such as DNS server component 118, may choose to resolve the future DNS query in advance.

[0044] With reference now to FIG. 5, upon receipt of the CNAME from the DNS server component 118, the client computing device 102 generates a subsequent DNS query corresponding to the CNAME. As previously discussed with regard to FIG. 4, the DNS query process could first start with DNS queries for the “.” and “com” portions, followed by a query for the “cdnprovider” portion of the CNAME. To the extent, however, that the results of a previous DNS queries can be cached (and remain valid), the client computing device 102 can utilize the cached information and does not need to repeat the entire process. However, at some point, depending on whether the CNAME provided by DNS server component 118 (FIG. 4) and the previous URL/CNAME share common CDN service provider domains, resolves to a different POP provided by the CDN service provider 106. As illustrated in FIG. 5, the DNS server component 124 of POP 122 is now authoritative based on the different information in the current CNAME previously provided by the DNS server component 118. As previously described, the DNS server component 124 can then determine whether to resolve the DNS query on the CNAME with an IP address of a cache component that will process the content request or whether to provide another alternative resource identifier selected in the manner described above.

[0045] For purposes of illustration, assume that the DNS server component 118 determines that the DNS query corresponding to the current CNAME (provided by DNS server component 116) also corresponds to a CNAME record in its data store. In such an example, the DNS server component 124 would do any necessary processing to select a specific CNAME and return the CNAME to the client computing device. With reference now to FIG. 6, the client computing device 102 would now transmit a second subsequent DNS query corresponding to the CNAME provided by DNS server component 124 (FIG. 5). In accordance with DNS query processes already described, the DNS query would illustratively be received by the DNS server component 130 of POP 128. Again, the DNS server component 130 can then determine whether to resolve the DNS query on the CNAME with an IP address of a cache component that will process the content request or whether to provide another alternative resource identifier selected in the manner described above. In this example, the DNS server component 130 returns an IP address.

[0046] In an illustrative embodiment, the DNS server components, such as DNS server component 130, can utilize a variety of information in selecting a resource cache component. In one example, the DNS server component can default to a selection of a resource cache component of the same POP. In another example, the DNS server components can select a resource cache component based on various load balancing or load sharing algorithms. Still further, the DNS server components can utilize network performance metrics or measurements to assign specific resource cache components. The IP address selected by a DNS server component may correspond to a specific caching server in the resource cache. Alternatively, the IP address can correspond to a hardware/software selection component (such as a load balancer).

[0047] With reference now to FIG. 7, in an illustrative example, assume that the DNS server component 130 has selected the resource cache component 132 of POP 128. Upon receipt of the IP address for the resource cache component 132, the client computing device 102 transmits requests for the requested content to the resource cache component 132. The resource cache component 132 processes the request in a manner described above and the requested content is transmitted to the client computing device 102.

[0048] With reference now to FIG. 8, a request routine 800 implemented by the CDN provider 106 will be described. One skilled in the relevant art will appreciate that actions/steps outlined for routine 800 may be implemented by one or many computing devices/components that are associated with the CDN service provider 106. Accordingly, routine 800 has been logically associated as being performed by the CDN service provider 106.

[0049] At block 802, one of the DNS server components 118, 124, 130 receives a DNS query corresponding to resource identifier. As previously discussed, the resource identifier can be a URL that has been embedded in content requested by the client computing device 102 and previously provided by the content provider 104. Alternatively, the resource identifier can also correspond to a CNAME provided by a content provider DNS server in response to a DNS query previously received from the client computing device 102. At decision block 804, a test is conducted to determine whether the current DNS server is authoritative to resolve the DNS query. In an illustrative embodiment, the DNS server can determine whether it is authoritative to resolve the DNS query if there are no CNAME records corresponding to the received resource identifier. Alternative or additional methodologies may also be practiced to determine whether the DNS server is authoritative.

[0050] If the current DNS server is authoritative (including a determination that the same DNS server will be authoritative for subsequent DNS queries), the current DNS server resolves the DNS query by returning the IP address of cache server component. In a non-limiting manner, a number of methodologies for selecting an appropriate resource cache component have been previously discussed. Additionally, as described above, the IP address may correspond to a specific cache server of a resource cache component or generally to group of cache servers.

[0051] Alternatively, if at decision block 804 the DNS server is not authoritative, at block 808, the DNS server component selects and transmits an alternative resource identifier. As described above, the DNS server component can utilize a data store to identify an appropriate CNAME as a function of the current DNS query. Additionally, the DNS server component can also implement additional logical processing to select from a set of potential CNAMES. At block 810, different DNS server components 118, 124, 130 receive a DNS query corresponding to the CNAME. The routine 800 then returns to decision block 804 and continues to repeat as appropriate.

[0052] While illustrative embodiments have been disclosed and discussed, one skilled in the relevant art will appreciate that additional or alternative embodiments may be implemented within the spirit and scope of the present invention. Additionally, although many embodiments have been indicated as illustrative, one skilled in the relevant art will appreciate that the illustrative embodiments do not need to be combined or implemented together. As such, some illustrative embodiments do not need to be utilized or implemented in accordance with scope of variations to the present disclosure.

[0053] Conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment.

[0054] Any process descriptions, elements, or blocks in the flow diagrams described herein and/or depicted in the attached figures should be understood as potentially representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or steps in the process. Alternate implementations are included within the scope of the embodiments described herein in which elements or functions may be deleted, executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those skilled in the art. It will further be appreciated that the data and/or components described above may be stored on a computer-readable medium and loaded into memory of the computing device using a drive mechanism associated with a computer readable storing the computer executable components such as a CD-ROM, DVD-ROM, or network interface further, the component and/or data can be included in a single device or distributed in any manner. Accordingly, general purpose computing devices may be configured to implement the processes, algorithms and methodology of the present disclosure with the processing and/or execution of the various data and/or components described above.

[0055] It should be emphasized that many variations and modifications may be made to the above-described embodiments, the elements of which are to be understood as being among other acceptable examples. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.