Abstract:
One embodiment of the present invention provides a system that facilitates reconfirmation of resource records in a cache. The system starts by retrieving the resource record at a client and attempting to establish communication using the data in that resource record. In the event that communication is not successful, the system responds by issuing one or more queries for the resource record in question. The system then waits for a response to the query. If the response to the query is not received in a pre-determined amount of time, the system invalidates the resource record. The response from the device is a multicast response, thereby allowing other clients to update corresponding resource records in their local caches without querying the device, and thus saving network bandwidth.

Description:
RELATED APPLICATION 
     The instant application is a divisional of, and hereby claims priority under 35 U.S.C. §120 to, pending U.S. patent application Ser. No. 10/676,572, entitled “Method and Apparatus for Accelerating the Expiration of Resource Records in a Local Cache,” by inventor Stuart D. Cheshire, which was filed on 30 Sep. 2003. U.S. patent application Ser. No. 10/676,572 claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application No. 60/496,843, filed on 20 Aug. 2003, entitled “Method and Apparatus for Accelerating the Expiration of Resource Records in a Local Cache,” by inventor Stuart D. Cheshire. 
    
    
     BACKGROUND 
     Field of the Invention 
     The present invention relates to computer networks. More specifically, the present invention relates to a method and an apparatus accelerating the expiration of local resource records in a local cache. 
     Related Art 
     The explosive growth of broadband technology and network infrastructure, coupled with continually decreasing prices, has led to an increasing number of computer networks in homes and small businesses. Along with this small-office/home-office network revolution comes the daunting task of configuring and administering these networks. Plug-and-play network protocols, such as Apple Computer Inc.&#39;s Rendezvous, have been developed to simplify administration tasks. Within Rendezvous, devices, such as printers, scanners, and network attached storage, automatically configure themselves and advertise their services upon being connected to the network. Computer systems on the network add resource records for these devices to their local cache upon receiving the advertisement, thus maintaining a list of available services on the network. 
     Along with the benefits of maintaining a local cache come problems. Computer systems on the network must continually update their cache records to maintain an accurate list of available services. Traditionally, computer systems have done this by querying the services and waiting for a response. If a response is not received within a specified amount of time, or after a specified amount of tries, the record pertaining to the service or associated device is deleted from the cache. Frequently polling services (and related devices) in this way provides a more accurate list of services, but can dramatically increase network traffic. 
     One problem with caching resource records arises from stale data. A device may be removed from the network in a manner that does not allow it to report the unavailability of its services. Power failures, improper shutdowns, and disconnected network cables are just a few examples of common occurrences that can cause invalid cache records. Often, users unsuccessfully try to use a service because a corresponding locally cached record for the service is invalid, which causes the service incorrectly to show up locally as being available. This can result in frustration to the user because the service still shows up as available in the local list. Moreover, even if some cache records are invalidated on some computer systems, the same invalid cache records may still be present on other computer systems on the network. 
     Hence, what is needed is a method and an apparatus for maintaining resource records in a cache without the limitations listed above. 
     SUMMARY 
     One embodiment of the present invention provides a system that facilitates validating a resource record in a cache. The system starts by retrieving the resource record at a client and attempting to use this information to establish communication with the corresponding service. If the attempt to establish communication is unsuccessful, then this suggests that the data in the resource record may be incorrect, so a query for that resource record is issued on the network. If a response is received containing different resource record data, then the cache is updated with the new data, and the attempt to establish communication is repeated using the new information. If, after sending the query one, two, three, or more, times, as appropriate, no response is received, then this indicates that the resource record is no longer valid, and it is deleted from the cache. In this way the stale data is deleted more promptly that it would be if the decision were made solely based on the time-to-live originally attached to that data. 
     When the record is deleted from the cache, this technique may then be applied recursively to call into question any records that refer to the now-deleted record. To take an example from the domain name system, if the now-deleted record were an address record, then any other records (e.g. SRV, PTR, CNAME, etc.) containing the name of this address record on their right-hand side are similarly suspect, and should be similarly updated or deleted as appropriate. In the case of Apple&#39;s Rendezvous, the response from the device is a multicast response, thereby allowing other clients to update corresponding resource records in their local caches without querying the device, and thus saving network bandwidth. 
     In a variation on this embodiment, prior to invalidating the resource record, the system issues a second query for the resource record and waits for a response to the second query from the device. If the response to the second query is not received in a pre-determined amount of time, the system invalidates the resource record. 
     In a variation on this embodiment, the system receives a message from a second client querying for a second resource record. Upon receiving the query, the system locates a second resource record and waits for a multicast response to the multicast message. If the multicast response to the multicast message is not received in the pre-determined amount of time, the system invalidates the second resource record. 
     In a variation on this embodiment, invalidating the resource record also involves invalidating a child record of the resource record. 
     In a variation on this embodiment, if the response to the query is not received in a pre-determined amount of time, the system retrieves a parent record of the resource record at the client and multicasts a query for the parent record. The system then waits for a response to the query from the device, and if the response to the query is not received in a pre-determined amount of time, the system invalidates the parent record. 
     In a variation on this embodiment, if the response to the query is not received in a pre-determined amount of time, the system retrieves a parent record of the resource record at the client and multicasts a query for the parent record. The system then receives a response to the query from the device, and updates the resource record with the information included in the response. 
     In a further variation, the system updates the parent record with the information received in the response. 
     In a variation on this embodiment, the system is invoked to validate resource records in the cache at a pre-specified time interval. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a computer network in accordance with an embodiment of the present invention. 
         FIG. 2  illustrates cached resource records in accordance with an embodiment of the present invention. 
         FIG. 3  presents a flowchart illustrating the process of updating a resource record in a cache in accordance with an embodiment of the present invention. 
         FIG. 4  presents a flowchart illustrating the process of updating a resource record in a cache in response to a query in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet. 
     Computer Network 
       FIG. 1  illustrates a computer network  100  in accordance with an embodiment of the present invention. Network  100  can generally include any type of wired or wireless communication channel capable of coupling together computing nodes. This includes, but is not limited to, a local area network, a wide area network, or a combination of networks. In one embodiment of the present invention, network  100  includes the Internet. 
     Network  100  includes computer systems  102  and  104 , printers  106  and  108 , and scanner  110 . Computer systems  102  and  104  can generally include any type of computer system, including, but not limited to, a computer system based on a microprocessor, a mainframe computer, a digital signal processor, a portable computing device, a personal organizer, a device controller, and a computational engine within an appliance. 
     In one embodiment of the present invention, the devices on network  100  adhere to a plug-and-play protocol, such as Apple Computer, Inc.&#39;s Rendezvous technology. Printers  106  and  108 , and scanner  110  self-configure to network  100  and advertise their services via multicast messages to all nodes on network  100 . Computer systems  102  and  104  each utilize a local cache that contains resource records for the known devices and services on network  100 . 
     Cached Resource Records 
       FIG. 2  illustrates cached resource records in accordance with an embodiment of the present invention. Each device on network  100 , such as printer  106 , may have a several resource records containing information pertaining to that device, such as service pointer record  202 , in the cache on computer systems in network  100 . Service pointer record  202  includes the type of service and the name of the service instance. In the example shown in  FIG. 2 , service pointer record  202  identifies a printing service named “Stuart&#39;s Printer.” 
     Each service pointer record  202  refers by name to a corresponding service record  204 , which contains information about a service on the device named in service record  204 . Note that one device may offer multiple services, and thus may have multiple service records. Service record  204  contains the name of the device that the service is provided by. In the example shown in  FIG. 2 , service record  204  contains the name of corresponding address record  206 , which contains the Internet Protocol (IP) address for the service. 
     Validating Resource Records in a Local Cache 
       FIG. 3  presents a flowchart illustrating the process of updating or invalidating resource records in a local cache in accordance with an embodiment of the present invention. The system starts when a request is received from a user to use a service on network  100  (step  302 ). Upon receiving the request, the system looks up the service record  204  for the device (step  304 ). 
     Next, the system determines the host address for connecting to the service provided by the resource (step  306 ). This is accomplished by looking up address record  206  that is referenced by service record  204 . Once the host address and port number are known, the system attempts to communicate with the service at the host and port number (step  308 ). If the communication is successful, i.e., if the service responds (step  310 ) (which is shown in  FIG. 3  as “service responds?”), the system uses the resource as requested by the user (step  312 ). 
     If the communication is not successful, i.e., if the service does not respond (step  310 ), the system marks address record  206  as suspect (step  316 ). The system then queries for the address record by sending one or more multicast messages to the entire network (step  318 ), and waiting for a response (step  320 ). If the service responds, the system updates address record  206  with information included in the response and uses the service as directed by the user (step  322 ). If no response is received after a specified amount of time and a specified amount of queries, the address record is deleted from the cache and the system marks service record  204  as suspect (step  324 ). In addition, the system invalidates child records of the resource record (step  325 ). 
     At this point, the process repeats and the system sends queries for the service record  204  by sending a multicast message to the entire network, and waiting for a response. If no response is received then service record  204  is deleted from the cache, and the system proceeds to reconfirm service pointer  202  in the same way. 
     In some embodiments, when a resource record associated with a device is invalidated, the system retrieves a parent record of the resource record at the client (the parent record referring to the resource record). The system then issues a parent-record query for the parent record and waits for a response to the parent-record query from the device. When a response to the parent-record query is not received in a predetermined time, the system invalidates the parent record. In some embodiments, when a given parent record is invalidated, the system recursively repeats, for ancestor records of the parent record, the process of: (1) retrieving an ancestor record, (2) querying for the ancestor record, and, if no response is received, (3) invalidating the ancestor record. 
     Second-Hand Updating of Resource Records in a Cache 
       FIG. 4  presents a flowchart illustrating the process of updating a resource record  202  in a local cache based on queries performed by other computer systems in accordance with an embodiment of the present invention. 
     The system starts when computer system  102  receives a multicast message from computer system  104 , wherein the multicast message queries a service provided by printer  106  (step  402 ). Computer system  102  looks in its local cache to locate service record  204  that corresponds to the query from computer system  104  (step  404 ). Next, computer system  102  waits for a response from the service (step  406 ). Note that in the present invention, devices and services respond to queries by sending a multicast message to network  100  rather than a unicast message directed only to the system that created the query. By sending a multicast response, this allows other devices on network  100  to update corresponding entries in their local caches without consuming extra bandwidth. If the service responds, computer system  102  updates corresponding resource records, such as address record  206  and service record  204 , or creates new versions of address record  206  and service record  204 , if necessary (step  408 ). If there is no response from the service, computer system  102  deletes service record  204  (step  410 ) as proceeds to reconfirm any records in the local cache which may refer to it. 
     The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.