Abstract:
A method and system for resolving domain name system (DNS) queries in a multiprotocol communications network is disclosed. The disclosed method includes in one embodiment receiving a destination address from a DNS server utilizing a first protocol; and communicating with a network element associated with the destination address utilizing a second protocol. In another embodiment, the disclosed method includes maintaining a profile for the DNS server and selecting the DNS server from a plurality of DNS servers utilizing the profile. In yet another embodiment, the disclosed method includes requesting a first address from the DNS server, where the first address is formatted according to a primary protocol, detecting a request failure in response to the request, and requesting a second address from the DNS server, where the second address is formatted according to a secondary protocol, in response to detecting the request failure.

Description:
BACKGROUND 
   1. Technical Field 
   The present invention relates to communication networks generally. More specifically, the present invention relates to a method and system for resolving Domain Name System (DNS) queries in a multiprotocol communications network. 
   2. Description of the Related Art 
   In many conventional communications networks, a domain name system (DNS) is used to translate between textual domain name strings often utilized as user labels for network elements (hosts, routers, etc.) and numerical addresses utilized to route data between source and destination nodes within communications network domains. A DNS system typically includes one or more DNS clients (e.g., an Internet browser client application) and one or more DNS servers (e.g., resolvers, name servers, etc.) arranged hierarchically within network elements of a communications network. 
   In many conventional communications networks, a single protocol, (e.g., IPv4) is implemented at the network layer level. As other network layer protocols, (e.g., IPv6) have been introduced, multiprotocol communications networks including network elements which implement any of two or more network-layer protocols exclusively and/or multiple protocols simultaneously have become more prevalent. 
     FIG. 1  is a high-level process flow diagram of a domain name system (DNS) query resolution process according to the prior art. In the illustrated process, a source network element first generates a domain name system (DNS) query for a destination network element&#39;s domain name (process block  102 ). As will be described in greater detail with respect to  FIG. 2 , a standard DNS query includes a destination or “target” domain name to be resolved, a query type specifying the type of resource record requested in the query, and query class. A determination is then made whether the query type of the generated query specifies a resource record associated with an IPv4 address (e.g., an A record) or an IPv6 address (e.g., a AAAA or A6 record) (process block  104 ). 
   Following a determination that an IPv4-type resource record type has been requested, the source network element transmits the generated DNS query to a DNS server utilizing an IPv4 transport (process block  106 A). Each DNS client is capable of contacting at least one DNS server (e.g., the name server for the DNS client&#39;s domain). DNS servers use a well-known protocol port for all communication, so clients may consequently communicate with a server once the address of the machine in which the name server executes is known. In some systems the address of the machine that supplies domain name service is bound into application programs at compile time while in others the address is configured into the operating system at startup. In still others systems, an administrator places the address of a name server in a file on secondary storage. 
   The DNS server receives the source network element-transmitted DNS query (process block  108 A) and attempts to resolve the query. A determination is then made whether or not the queried DNS server includes an IPv4 address resource record (A record) corresponding to the query-specified domain name (process block  110 A). If so, the DNS server provides the IPv4 address to the source network element (process block  112 A). The source network element then receives the IPv4 address (process block  114 A) and utilizes it to communicate with the destination network element (process block  116 A). According to the prior art, the transmission of IPv4 addresses from the DNS server to the source network element (process block  112 A) and communication between the source and destination network elements (process block  116 A) is performed using the same transport type as was utilized to transmit the DNS query to the DNS server (process block  106 A) (i.e., IPv4). 
   If a determination is made that the queried DNS server does not include an IPv4 address resource record (an A record) corresponding to the query-specified domain name, the DNS server indicates to the source network element that the queried name is unknown utilizing an IPv4 transport (process block  118 A) and the process of the illustrated embodiment is terminated. In an alternative prior art embodiment not illustrated by  FIG. 1 , if a determination is made that the IP address resource record cannot be provided the DNS server determines whether the DNS query is recursive or iterative. For a recursive DNS query, the DNS server recursively queries other DNS servers to resolve the query and then provides the resolved IPv4 address to the source network element. For an iterative DNS query, the DNS server generates a reply utilizing an IPv4 transport which specifies another DNS server that the source network element should contact. 
   Following a determination that an IPv6-type resource record has been requested, a procedure (process blocks  106 B- 118 B) paralleling that described with respect to process blocks  106 A- 118 A is performed. As with the previously described procedure, the transmission of IPv6 addresses from the DNS server to the source network element (process block  112 B) and communication between the source and destination network elements (process block  116 B) is performed using the same transport type, (IPv6 here), as that utilized to transmit the DNS query to the DNS server (process block  106 B). 
   As will be apparent from the preceding description, conventional methods of resolving DNS queries within multiprotocol (e.g., mixed IPv4 and IPv6) communications networks suffer from several drawbacks. One such drawback, illustrated by  FIG. 1 , is that such conventional methods require that the same protocol (i.e., either IPv4 or IPv6) is utilized to perform a DNS query as will be utilized to communicate with the destination network element which is the subject of the DNS query. Consequently, resource records (e.g., A records for IPv4 addresses and AAAA or A6 records for IPv6 addresses) associated with a given protocol are prevented from being usefully stored within and accessed from a DNS server which is not compatible with that particular protocol, making a transition from IPv4 to IPv6 more difficult and costly. 
   Another significant drawback associated with conventional DNS query resolution methods is that in using such methods, it is typically presumed that no DNS server is any more likely or capable of resolving a DNS query than any other DNS server. Accordingly, in generating iterative DNS queries, a DNS client software application (or DNS server attempting to resolve a recursive DNS client query) will typically query each DNS server it has access to in turn in order to resolve a DNS query. Although the performance penalty associated with this technique is relatively small for the majority of network elements (e.g., hosts) which have access to only one primary and possibly one secondary DNS server, it may be substantially larger for other network elements (e.g., network elements within a corporate communications network, routers, etc.) having access to a greater number of DNS servers. 
   SUMMARY OF THE INVENTION 
   A method and system for resolving domain name system queries in a multiprotocol communications network is disclosed. The disclosed method includes receiving a destination address from a domain name system (DNS) server utilizing a first protocol (e.g., a first network-layer protocol); and communicating with a network element associated with the destination address utilizing a second protocol (e.g., a second network-layer protocol). 
   In another embodiment, the disclosed method includes maintaining a profile of each known DNS server and selecting the DNS server from a plurality of DNS servers utilizing the profile. 
   In yet another embodiment, one of the first protocol and the second protocol is a primary protocol and the other is a secondary protocol and the disclosed method includes requesting a first address from the DNS server, where the first address is formatted according to the primary protocol, detecting a request failure in response to the request, and requesting a second address from the DNS server, where the second address is formatted according to the secondary protocol, in response to detecting the request failure. 
   The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. As will also be apparent to one of skill in the art, the operations disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings in which similar references are utilized to indicate similar elements and in which: 
       FIG. 1  is a high-level process flow diagram of a domain name system (DNS) query resolution process according to the prior art; 
       FIG. 2  is a block diagram of a DNS server query message useable with one or more embodiments of the present invention; 
       FIG. 3A  is a high-level process flow diagram of a first portion of a domain name system (DNS) query resolution process according to an embodiment of the present invention; 
       FIG. 3B  is a high-level process flow diagram of a second portion of a domain name system (DNS) query resolution process according to an embodiment of the present invention; 
       FIG. 4  is a block diagram of a communications network useable with one or more embodiments of the present invention; 
       FIG. 5  is a block diagram of an exemplary network element data processing system according to an embodiment of the present invention; 
       FIG. 6  is a conceptual block diagram of a memory space of a DNS server network element data processing system according to one embodiment of the present invention; 
       FIG. 7  is a block diagram of a resource record useable with one or more embodiments of the present invention; 
       FIG. 8  is a conceptual block diagram of a memory space of a DNS client network element data processing system according to one embodiment of the present invention; 
       FIG. 9  is a block diagram of DNS server profiling data structure according to one embodiment of the present invention; and 
       FIG. 10  is a high-level process flow diagram of a DNS server profiling process according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention may include features or processes embodied within machine-executable instructions provided by a machine-accessible medium. Such a medium may include any mechanism which provides (i.e., stores and/or transmits) data in a form accessible by a machine (e.g., a data processing system, host, router, or other network element, etc.). For example, a machine-accessible medium may include volatile and/or non-volatile media (e.g., read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc.), as well as electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc. 
   Such instructions can be used to cause a general or special purpose processor, programmed with the instructions, to perform processes of the present invention. Alternatively, processes of the present invention may be performed by specific hardware components containing hard-wired logic to perform operations or by any combination of programmed data processing components and hardware components. Thus, embodiments of the present invention may include software, data processing hardware, data processing system-implemented methods, and various processing operations, further described herein. 
     FIG. 2  is a block diagram of a domain name system (DNS) server query message useable with one or more embodiments of the present invention. The illustrated DNS server query message includes a header having a unique IDENTIFICATION field which a client may use to match responses to queries, and a PARAMETER field which may be utilized to specify a requested operation and response code. According to one embodiment, the illustrated PARAMETER field includes an operation bit field specifying whether the message is a query or a response, one or more query type bit fields specifying the kind of query in the message (e.g., a standard query, inverse query, server status request, etc.), an authoritative resolution bit field specifying whether the responding DNS server is an authority for the domain in question, a truncation bit field specifying whether the message was truncated due to length greater than that permitted on the transmission channel, a recursion desired bit field which may be set in a query and copied in the response to direct the name server to pursue a query recursively, a recursion available bit field which may be set in a response to denote whether recursive query support is available in the DNS server, and a response type bit field which may be set in responses to indicate various types of errors that may have occurred. 
   Also included within the header section of the illustrated server query message format are a number of questions field specifying the number of questions within the question section, a number of answers field specifying the number of resource records in the answer section, a number of authority field specifying the number of resource records in the authority records section, and a number of additional field specifying the number of resource records in the additional records section. According to one embodiment of the present invention, each query contained within the question section of a DNS server query message includes a query domain name followed by query type and query class fields described herein. 
     FIG. 3A  is a high-level process flow diagram of a first portion of a domain name system (DNS) query resolution process according to an embodiment of the present invention. In the illustrated process embodiment of  FIGS. 3A and 3B , a transitionary DNS strategy is employed for use in multiprotocol communications networks in which an IPv4-type address (e.g., an A resource record) is requested only after an IPv6-type address (e.g., AAAA and A6 resource records) has been requested and the request has failed. 
   Accordingly, a determination is made at the beginning of the portion of the DNS query resolution process illustrated in  FIG. 3A  whether there are any accessible DNS servers to which a request for an IPv6-type address has not yet been made (process block  302 ). If not, a corresponding IPv4-type address is requested in the portion of the DNS query resolution process illustrated in  FIG. 3B . Otherwise, another DNS server is selected (process block  304 ) and a determination is made whether the selected DNS server is configured to be accessed utilizing an IPv6 or IPv4 address as illustrated by process blocks  306  and  318 , respectively. If the selected DNS server is not accessible via an IPv6 or IPv4 address an error is reported (process block  330 ) and the illustrated process embodiment is terminated. 
   If the selected DNS server is accessible utilizing an IPv6 address, an IPv6 transport is utilized to request an AAAA record (process block  308 ). It will be readily appreciated that alternative embodiments of the invention may be practiced in which any other alternative resource record type(s) (e.g., A6, Canonical name, Mail Exchanger, Name Server, Pointer, Start of Authority, etc.) may be substituted for the requested AAAA record. Thereafter, the illustrated process embodiment continues until a response to the DNS query is received (process block  310 ) or a timeout occurs (process block  316 ). If a response is received, a determination is made whether or not the response includes the requested AAAA resource record (process block  312 ). If either the requested resource record is not returned or a timeout occurs, the illustrated process embodiment is restarted as additional DNS servers to query are sought (process block  302 ). Alternatively, a communication session is begun utilizing the returned AAAA resource record over an IPv6 transport (process block  314 ) before terminating the illustrated process embodiment. 
   If the selected DNS server is accessible utilizing an IPv4 address, an IPv4 transport is utilized to request an AAAA record (process block  320 ) in contrast to conventional DNS query resolution methods in which only an IPv6 transport may be utilized to request an IPv6-type address such as an AAAA record. Thereafter, the illustrated process embodiment continues until a response to the DNS query is received (process block  322 ) or a timeout occurs (process block  328 ). If a response is received, a determination is made whether or not the response includes the requested AAAA resource record (process block  324 ). If either the requested resource record is not returned or a timeout occurs, the illustrated process embodiment is restarted as additional DNS servers to query are sought (process block  302 ). Alternatively, a communication session is begun utilizing the returned AAAA resource record over an IPv6 transport (process block  326 ) before terminating the illustrated process embodiment. 
     FIG. 3B  is a high-level process flow diagram of a second portion of a domain name system (DNS) query resolution process according to an embodiment of the present invention. To the extent that the portion of the DNS query resolution process embodiment illustrated by  FIG. 3B  is only performed if no corresponding IPv6-type address may be retrieved from an accessible DNS server it may be considered optional. As with the first portion of the DNS query resolution process illustrated by  FIG. 3A , a determination is first made in the illustrated process embodiment whether there are any accessible DNS servers to which a request for an IPv4-type address has not yet been made (process block  332 ). If not, the DNS query resolution process fails and the illustrated process embodiment is terminated. Otherwise, another DNS server is selected (process block  334 ) and a determination is made whether the selected DNS server is configured to be accessed utilizing an IPv6 or IPv4 address as illustrated by process blocks  336  and  348 , respectively. If the selected DNS server is not accessible via an IPv6 or an IPv4 address an error is reported (process block  360 ) and the illustrated process embodiment is terminated. 
   If the selected DNS server is accessible utilizing an IPv6 address, an IPv6 transport is utilized to request an A record (process block  338 ) in contrast to conventional DNS query resolution methods in which only an IPv4 transport is utilized to request an IPv4-type address such as an A record. Thereafter, the illustrated process embodiment continues until a response to the DNS query is received (process block  340 ) or a timeout occurs (process block  346 ). If a response is received, a determination is made whether or not the response includes the requested A resource record (process block  342 ). If either the requested resource record is not returned or a timeout occurs, the illustrated process embodiment is restarted as additional DNS servers to query are sought (process block  332 ). Alternatively, a communication session is begun utilizing the returned A resource record over an IPv4 transport (process block  344 ) before terminating the illustrated process embodiment. 
   If the selected DNS server is accessible utilizing an IPv4 address, an IPv4 transport is utilized to request an A record (process block  350 ). Thereafter, the illustrated process embodiment continues until a response to the DNS query is received (process block  352 ) or a timeout occurs (process block  358 ). If a response is received, a determination is made whether or not the response includes the requested A resource record (process block  354 ). If either the requested resource record is not returned or a timeout occurs, the illustrated process embodiment is restarted as additional DNS servers to query are sought (process block  332 ). Alternatively, a communication session is begun utilizing the returned A resource record over an IPv4 transport (process block  356 ) before terminating the illustrated process embodiment. 
   In one alternative embodiment of the present invention, a query is generated and transmitted substantially in parallel to a selected DNS server for each of a number of resource record types (e.g., for a type A and type AAAA resource record) rather than serially as described herein with respect to process blocks  308 ,  320 ,  338  and  350  of  FIGS. 3A and 3B . In the described embodiment, DNS server responses, if any, for one resource record type (e.g., type A records) may be cached and utilized only upon a determination that no resource records of the other type (e.g., type AAAA records) were available. In another similar embodiment, such parallel DNS queries are transmitted by a DNS client to each of its available DNS servers substantially simultaneously with the responses being parsed and utilized upon receipt as described herein (e.g., caching and utilizing type A records when no AAAA type records are available). 
     FIG. 4  is a block diagram of a communications network useable with one or more embodiments of the present invention. The communications network  400  of the illustrated embodiment includes a source network element  402 A, a destination network element  402 B, and a DNS server  404  coupled together via a communication medium  406  (e.g., an Ethernet, token ring, Fiber Distributed Data Interface, etc.). According to an embodiment of the present invention, DNS server  404  and source and destination network elements  402 A and  402 B are implemented within data processing systems (e.g., hosts). In the illustrated embodiment, source network element  402 A is a multiprotocol or “dual-stack” node, capable of operating utilizing multiple network-layer protocols, (e.g., IPv4 and IPv6). In another embodiment of the present invention source network element  402 A performs the process described with respect to  FIGS. 3A and 3B  to perform a DNS lookup from DNS server  404  on, and subsequently communicate with, destination network element  402 B over communication medium  406 . 
     FIG. 5  is a block diagram of a network element data processing system (e.g. a DNS server and/or DNS client) according to an embodiment of the present invention. The data processing system of the illustrated embodiment includes a processor  500  and a memory  502  communicatively coupled together via a bus  504  or other communication interconnect and further includes a communications port  506  (e.g., a modem, network interface, etc.) as shown. According to one embodiment of the present invention, memory  502  serves as a machine-readable medium as described herein, providing data and instructions to and receiving processed data from processor  500  via bus  504 . 
   Data processing systems according to alternative embodiments of the present invention may include additional elements and/or features. For example, according to one embodiment, a data processing system may include various input/output devices (e.g., keyboards, cursor control devices, displays, printers, scanners, etc.), additional processors, memories, buses, and the like. Similarly, a data processing system according to an embodiment of the present invention may be embodied within a wide variety of data processing devices (e.g., personal computers, workstations, servers, thin clients, routers, gateways, personal digital assistants, etc.). 
     FIG. 6  is a conceptual block diagram of a memory space of a domain name system (DNS) server network element data processing system according to one embodiment of the present invention. Memory space  600  of the illustrated embodiment includes a first memory region  602  to store resource records such as A resource records  604 , AAAA resource records  606 , A6 resource records  608 , etc. as well as a second memory region  610  to store a DNS server application program to perform features, methods, and process operations of various embodiments of the present invention. 
     FIG. 7  is a block diagram of a resource record useable with one or more embodiments of the present invention. For example, according an embodiment of the present invention the illustrated resource record comprises an address resource record (e.g., a type A, AAAA, or A6 resource record). According to another embodiment, the RESOURCE DOMAIN NAME field contains the domain name to which this resource record refers; the TYPE field specifies the type of data included in the resource record; the CLASS field specifies the data&#39;s class; the TIME TO LIVE field contains a number specifying the number of seconds information in this resource record can be cached which may be used, for example, by DNS clients who have requested a name binding which want to cache the results. Also included with the depicted resource record is a RESOURCE DATA LENGTH field specifying the number of octets in a RESOURCE DATA field. 
     FIG. 8  is a conceptual block diagram of a memory space of a DNS client network element data processing system according to one embodiment of the present invention. Memory space  800  of the illustrated embodiment includes a first memory region  802  to store a server profiling data structure as well as a second memory region  804  to store a DNS client application program to perform features, methods, and process operations of various embodiments of the present invention. 
     FIG. 9  is a block diagram of DNS server profiling data structure according to one embodiment of the present invention. The DNS server profiling data structure or “DNS server scoreboard” of the illustrated embodiment includes information utilized in the selection of DNS servers as described with respect to process blocks  304  and  334  of  FIGS. 3A and 3B , respectively, according to one embodiment of the present invention. In the illustrated embodiment, the DNS server profiling data structure includes an entry for each DNS server known to the DNS client. Each entry in turn includes a unique DNS server identifier and a score for each type of resource record which may be requested by the client representing the identified DNS server&#39;s relative ability to fulfill requests for resource records of that type. In the illustrated embodiment, each score is represented by a numeric value equal to or greater than zero. The generation and maintenance of the score values depicted according to one embodiment of the present invention will be described in greater detail with respect to  FIG. 10 . 
     FIG. 10  is a high-level process flow diagram of a DNS server profiling process according to an embodiment of the present invention. In one embodiment, the illustrated process is performed by a DNS client which maintains a DNS server profiling data structure such as that described with respect to  FIG. 9 . In other alternative embodiments, each DNS client within a communications network may determine whether or not to maintain such a data structure and perform the illustrated process based upon a number of factors (e.g., user selection, the number of DNS servers available to the DNS client, the penalty associated with selecting the incorrect DNS server for DNS query resolution, etc.). When the illustrated process is performed, the resource record type being requested is determined (process block  1000 ) and an internal list, maintained by the DNS client, of the DNS servers which have been queried is cleared (process block  1002 ). 
   Thereafter, the DNS client uses the DNS server profiling data structure to select the DNS server with the highest score value for the requested resource record type from the DNS servers which have not already been queried (process block  1004 ), adds the selected DNS server to the list of queried servers (process block  1006 ), sends the DNS query to the selected DNS server (process block  1008 ), and waits for a reply (process block  1010 ). Once a reply has been received, a determination is made whether the reply was for the queried resource record type (process block  1012 ). If so, the score value for the selected DNS server is increased (process block  1014 ) and the illustrated process is terminated. Otherwise, the score value for the selected DNS server is decreased for the requested resource record type (process block  1016 ) as shown. Once the DNS server&#39;s score value has been appropriately decreased, a determination is made whether any available servers have not been queried for the desired resource record (process block  1018 ). If such DNS servers are available, a DNS server is selected from those remaining with the highest score value for the requested resource record type (process block  1004 ) and the illustrated process continues from that point as described. 
   Consequently, the score value of DNS servers which are capable of resolving queries for particular resource record types are increased, thus increasing the likelihood that those DNS servers will be selected as the first DNS server to be queried. Conversely, the score values of DNS servers which are incapable of resolving queries for resource records of a particular type are decreased, lessening the chance that such DNS servers will be selected initially in the future. While score values have been illustrated herein with respect to  FIG. 9  as numeric values equal to or greater than zero, other mechanisms for profiling DNS server ability to resolve queries for resource records of a particular type have been contemplated in alternative embodiments of the invention. 
   According to one such embodiment, score values may include negative numbers. In another embodiment, upper and/or lower limits on score values are enforced, with the score value being reset to a default value after the limit has been exceeded. According to other embodiments of the present invention, a user may be given the capability to set such limits and/or to reset the DNS server profiling data structure (e.g., when a new DNS server is added or made available within a communications network). In another embodiment, score values having one of two values representing a preferred or deprecated state may be implemented. Available DNS servers may also simply be ordered or prioritized utilizing unique score values of 1 to the number of available DNS servers. 
   According to another embodiment of the present invention, the amount that a score value is increased or decreased may be varied from one (e.g., different increase and decrease factors and/or different factors based upon the type of resource record requested, the DNS server&#39;s prior score value or other circumstances of the request). In another embodiment, a DNS client may cease generating queries for resource records of a particular type and/or generate queries exclusively for resource records of a particular type in response to a score value exceeding a predetermined threshold, a predetermined number of queries being generated without being resolved, etc. 
   In the foregoing description, the present invention has been described with reference to specific exemplary embodiments thereof. It should be understood however that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense.