Patent Publication Number: US-7904576-B2

Title: Reliable datagram via independent source destination resources

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Utility Patent Application is a Continuation of U.S. application Ser. No. 09/578,155 filed on May 23, 2000 now U.S. Pat. No. 7,318,102, which claims the benefit of the filing date of U.S. Provisional Applications Ser. No. 60/135,664, filed May 24, 1999 and U.S. Ser. No. 60/154,150, filed Sep. 15, 1999, all of which are herein incorporated by reference. 
    
    
     THE FIELD OF THE INVENTION 
     The present invention generally relates to data processing, and more particularly to communication between distributed application instances via a reliable datagram service. 
     BACKGROUND OF THE INVENTION 
     In conventional data processing systems, distributed application instances typically employ transport services, such as a reliable connection service or an unreliable datagram service, to communicate. An application instance is herein defined to be a producer or a consumer of data in the data processing system. An application instance can be implemented in software, hardware, or firmware, or in any combination of these. A unit of work is herein defined to be data which is transmitted between a source application instance and a destination application instance. Accordingly, a source application instance is the producer of the unit of work sent to the destination application instance. The destination application instance is the consumer of the unit of work sent from the source application instance. 
     A portion of a conventional data processing system employing a reliable connection service to communicate between distributed application instances is illustrated generally at  30  in  FIG. 1 . Conventional data processing system  30  includes an application instance A indicated at  32 , an application instance B indicated at  34 , and an application instance C indicated at  36 . The reliable connection service of data processing system  30  creates at least one non-sharable resource connection between each connected pair of communicating application instances. For example, a first non-sharable resource connection  38  and a second non-sharable resource connection  40  are created between application instance A indicated at  32  and application instance B indicated at  34 . A third non-sharable resource connection is created between application instance A indicated at  32  and application instance C indicated at  36 . Each non-sharable resource connection includes a unique set of non-sharable resources. The reliable connection service transmits units of work between application instances by identifying a source connection handle and by issuing appropriate instructions to control data transmission. Reliable connection services provide reliable communication between application instances, but at the cost of scalability of the data processing system. In reliable connection services, communication at any one time is restricted to one-to-one application instance relationships via corresponding non-sharable resource connections. 
     A portion of a conventional data processing system employing an unreliable datagram service to communicate between application instances is illustrated generally at  50  in  FIG. 2 . Conventional data processing system  50  includes an application instance A indicated at  52 , an application instance B indicated at  54 , and an application instance C indicated at  56 , and an application instance D indicated at  58 . The unreliable datagram service employed by data processing system  50  creates a shared resource datagram  60 . Shared resource datagram  60  can be employed to transmit units of work between multiple application instances. Shared resource datagram  60  couples application instance A indicated at  52  to application instance B indicated at  54 , to application instance C indicated at  56 , and to application instance D indicated at  58 . Unreliable datagram services provide for highly scalable data processing systems, but at the cost of reliability. In an unreliable datagram service, the application instance relationships can be one-to-one, one-to-many, or many-to-one, but communication between application instances is not reliable. In particular, traditional unreliable datagrams do not provide guaranteed ordering of units of work transmitted between application instances. 
     For reasons stated above and for other reasons presented in greater detail in the Description of the Preferred Embodiments section of the present specification, there is a need for an improved transport service for communicating between distributed application instances in data processing systems. The improved transport service should provide reliable communication between application instances including guaranteed ordering of units of work transmitted between application instances. In addition, the improved transport service should provide for highly scalable data processing systems. 
     SUMMARY OF THE INVENTION 
     One embodiment provides a method of processing data including producing units of work with at least one source application instance (AI) at a source device and consuming units of work with at least one destination AI at a destination device. The method includes establishing a first reliable datagram service, with a first source and destination resource (SDR), between the source device and the destination device and establishing a second reliable datagram service, with a second SDR independent of the first SDR, between the source device and the destination device. The method includes transmitting a first unit of work stream over a communication services/fabric with the first reliable datagram service and guaranteeing strong ordering of the first unit of work stream received at the destination device with the first reliable datagram service. The method includes transmitting a second unit of work stream over the communication services/fabric with the second reliable datagram service, and guaranteeing strong ordering of the second unit of work stream received at the destination device with the second reliable datagram service. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a conventional data processing system employing a reliable connection service to communicate between distributed application instances. 
         FIG. 2  is a diagram of a portion of a conventional data processing system employing an unreliable datagram service to communicate between application instances. 
         FIG. 3  is a diagram of a portion of a data processing system according to the present invention employing a reliable datagram service for providing reliable communication between distributed application instances. 
         FIG. 4  is a diagram of a portion of a data processing system according to the present invention having multiple source and destination resources (SDRs) for implementing a reliable datagram service between multiple devices. 
         FIG. 5  is a diagram of one embodiment of a protocol header according to the present invention containing fields employed an underlying communication services/fabric to target a destination application instance in a reliable datagram service according to the present invention. 
         FIG. 6  is a diagram illustrating an example transmission operation between a source device and a destination device for the data processing system of  FIG. 4 . 
         FIG. 7  is a diagram illustrating another example transmission operation between a source device and a destination device for the data processing system of  FIG. 4 . 
         FIG. 8  is a diagram illustrating another example transmission operation between a source device and a destination device for the data processing system of  FIG. 4 . 
         FIG. 9  is a diagram of a portion of a data processing system according to the present invention having multiple SDRs established between device pairs. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     A portion of a data processing system according to the present invention is illustrated generally at  100  in  FIG. 3 . Data processing system  100  includes a reliable datagram service according to the present invention for providing reliable communication between distributed application instances (AIs). The reliable datagram service according to the present invention also provides for a highly scalable data processing system  100 . 
     Data processing system  100  includes a source and destination resource (SDR)  102  for implementing a reliable datagram service between a device  104  and a device  106 . SDR  102  comprises SDR resources  102   a  at device  104  and SDR resources  102   b  at device  106 . SDR resources  102   a  communicate with SDR resources  102   b  via a communication services/fabric  108 . Device  104  and device  106  can each be a source or a destination device depending on the direction of communication. Device  104  includes an AI  110 , an AI  112 , and an AI  114 . Device  106  includes an AI  116 , an AI  118 , and an AI  120 . AIs produce or consume data. AIs can be implemented in software, hardware, or firmware, or in any combination of these. 
     A first step in implementing a reliable datagram service between source and destination devices, such as devices  104  and  106 , is to create a SDR according to the present invention, such as SDR  102 , upon which reliable communication can be implemented in a scalable data processing system. A variety of suitable techniques can be used to create a SDR according to the present invention. One such suitable technique employs an unreliable datagram service between middleware AIs on the source and destination devices, such as middleware AI  122  on device  104  and middleware AI  124  on device  106 . The middleware AIs exchange sufficient data to uniquely identify the SDR which will be employed to exchange reliable datagrams. Middleware AIs facilitate communication between AIs either directly, such as by providing a communication application programming interface (API) and library, or indirectly, such as by providing transparent error recovery and other management services. Middleware AIs can be implemented in hardware via a state machine, in firmware, or in software within a given device, such as devices  104  and  106 . One embodiment of a middleware AI (e.g., middleware AIs  122  and  124 ) operates local to the AIs within the same device. One embodiment of a middleware AI, such as middleware AI  126 , executes remotely and is accessed via the underlying communication services/fabric. 
     Once a SDR according to the present invention is established, any number of source AIs can employ the established SDR, such as SDR  102 , to communicate to any number of destination AIs, because the established SDR functions as a point of multiplexing at the source device and as a point of demultiplixing at the destination device. 
     In an example operation of SDR  102 , device  104  is a source device and device  106  is a destination device. In this example operation, AIs  110 ,  112 , and  114  of source device  104  are source AIs which produce units of work transmitted to the destination device  106 . In this example operation, AIs  116 ,  118 , and  120  are destination AIs which consume the units of work transmitted from corresponding source AIs  110 ,  112 , and  114 . In this example operation, SDR resources  102   a  at source device  104  multiplex units of work produced by source AIs  110 ,  112 , and  114  into a serial unit of work stream provided on communication services/fabric  108 . The serial unit of work stream is demultiplexed by SDR resources  102   b  at destination device  106  into units of work consumed by AIs  116 ,  118 , and  120 . In this example operation, SDR resources  102   b  validate the delivery of units of work, generate positive acknowledgements (ACKs) and negative acknowledgments (NAKs), and perform resynchronization operations based on any detected errors. 
     The reliable datagram service implemented with a SDR, such as SDR  102 , provides for distributed AI communication using one-to-one, one-to-many, or many-to-one AI relationships. In addition, AIs can operate within the same device via a device backplane fabric (e.g., a form of shared memory within a device) or between disparate devices which are connected via an intermediate communication fabric. Therefore, the communications between AIs is independent of the physical locality of the AIs and is connectionless from the perspective of the AIs. 
     A unit of work is data transmitted between a source AI and a destination AI. In one embodiment, the units of work are treated as opaque objects by the underlying communication services/fabric. In one embodiment, however, the underlying communication services/fabric performs optional filtering services on the units of work based on the capability of the underlying communication services/fabric and the requirements of the middleware AIs and AIs. 
     The reliable datagram service according to the present invention includes the following mechanisms to assure reliable transmission of the units of work between the source AIs and the destination AIs. A strong ordering mechanism in the SDR guarantees that the destination AIs receive the units of work in the same order that the corresponding source AIs sent the unit of work. Units of work sent by other source AIs to the same destination AI using separate SDRs may be interleaved. Strong ordering is only guaranteed on a one-to-one source AI-to-destination AI resource basis. 
     A second mechanism for assuring reliable transmission of the units of work between the source AIs and the destination AIs is that a given unit of work is received by the destination SDR resources exactly once. In other words, duplicate copies of units of work which may be generated during an error event or a recovery operation are detected and not delivered to the destination AI. 
     A third mechanism for assuring reliable transmission of the units of work between the source AIs and the destination AIs is an acknowledgement mechanism. The source AI and/or the communication services/fabric are informed of a unit of work transmission completion either via a positive acknowledgement (ACK) which indicates a unit of work was successively transmitted and received or a negative acknowledgement (NAK) which indicates an unrecoverable error was detected either within the unit of work or in its transmission. In one embodiment, a source AI is notified of an error through out-of-band communication, such as an alarm generated by the underlying communication services/fabric when a hardware failure is detected. 
     In one embodiment, AIs use the same sender-based or receiver-based communication and memory management/protection techniques as traditionally used by reliable connections services. In addition, AIs can implement other reliable operations and additional reliable techniques utilizing reliable datagrams which are not normally implemented using reliable connections. 
     Logical unit of work transmission size is limited only by the size of the memory window exported (sender-based communication) or posted (receiver-based communication) by the destination AI. Receiver-based AIs can support one or multiple memory buffers as transmission targets. The receiver communication services/fabric are responsible for selecting the optimal memory buffer to target for a given unit of work based on unit of work attributes or AI input. 
     Memory is protected using techniques such as Hamlyn protection mechanisms to ensure correct access rights (e.g., no access, read access, write access, read/write access, memory address range verification, and the like) are verified before access is granted to an AI. 
     A portion of a data processing system according to the present invention having multiple SDRs is illustrated generally at  200  in  FIG. 4 . Data processing system  200  includes a device A indicated at  202 , a device B indicated at  204 , a device C indicated at  206 , and a device D indicated at  208 . A SDR  210  including SDR resources  210   a  at device  202  and SDR resources  210   b  at device  204  implements a reliable datagram service between device  202  and device  204 . A SDR  212  including SDR resources  212   a  at device  202  and SDR resources  212   b  at device  206  implements a reliable datagram service between device  202  and  206 . A SDR  214  including SDR resources  214   a  at device  204  and SDR resources  214   b  at device  208  implements a reliable datagram service between device  204  and  208 . A SDR  216  including SDR resources  216   a  at device  206  and SDR resources  216   b  at device  208  implements a reliable datagram service between device  206  and device  208 . The source and destination ends of each of the SDRs  210 ,  212 ,  214 , and  216  communicate via a communication services/fabric  218 . Devices  202 ,  204 ,  206 , and  208  can each be a source or a destination device depending on the direction of communication. 
     Device  202  includes an AI  220  coupled to SDR resources  210   a  and SDR resources  212   a ; and an AI  222  coupled to SDR resources  212   a . Device  204  includes an AI  224  coupled to SDR resources  210   b ; and an AI  226  coupled to SDR resources  210   b  and SDR resources  214   a . Device  206  includes an AI  228  coupled to SDR resources  212   b ; and an AI  230  coupled to SDR resources  216   a.  Device  208  includes an AI  232  coupled to SDR resources  214   b ; and an AI  234  coupled to SDR resources  214   b  and SDR resources  216   b.    
     Thus, for each &lt;source, destination&gt; device tuple, one or more SDRs are established (e.g., SDR  210  is established for the device  202 -device  204  tuple). The AIs on each device can communicate to any AI on another device to which they are coupled through an associated SDR. One or more SDRs can be associated with a given physical fabric device allowing traffic segregation and multiple topologies to be supported. 
     Example Reliable Communication Protocol 
     The above-described strong ordering mechanism, the mechanism providing that a given unit of work is received by the destination SDR resources exactly once, and the acknowledge mechanism, which together assure reliable transmission of the units of work between the source AIs and the destination AIs, can be implemented according to the following example reliable communication protocol. 
     The example reliable communication protocol includes serializing all units of work which are transmitted between a source AI and a corresponding destination AI into a unit of work stream. The serialization of the units of work into a unit of stream is accomplished as follows. In one embodiment, each unit of work is treated as an opaque object which is encapsulated within a protocol header for transmission. A protocol header contains the fields employed by the underlying communication services/fabric to target the destination AI. 
     One example embodiment of a protocol header is illustrated generally at  300  in  FIG. 5 . Protocol header  300  includes target fields  302  which are employed by the underlying communication services/fabric to target the destination AI. A SDR identifier field  304  uniquely identifies the SDR employed to send and receive the units of work. All communication requiring strong ordering must flow through the same SDR. Strong ordering is not guaranteed across SDRs within the same device. 
     A SDR sequence number field  306  provides a unique logical time stamp representing a defined order of the units of work in the unit of work stream transmitted from the source SDR resources and is employed by the destination SDR resources for verifying that units of work are arriving in order and for detecting if any units of work are missing. The successive protocol headers in a given serial unit of work stream contain monotonically increasing number values in their respective SDR sequence number fields, which are assigned per SDR. 
     If the SDR sequence number field  306  value matches the expected sequence number valve stored in the destination SDR resources, then other protocol header  300  fields are verified by the destination SDR resources. The destination SDR resources provide an ACK if the current unit of work is determined to be valid from the destination device&#39;s perspective. The destination SDR resources provide a NAK if the current unit of work is determined to be invalid from the destination device&#39;s perspective. 
     The SDR sequence number field  306  value being less than an expected next sequence number value stored in the destination SDR resources indicates that the unit of work is a duplicate unit of work and the unit of work is dropped by the destination SDR resources. In one embodiment, the duplicate unit of work is silently dropped by the destination SDR resources. In one embodiment, the destination SDR resources drop the duplicate unit of work and provide an ACK indicating to the source SDR resources that the last unit of work was received to avoid the source SDR resources from again transmitting the duplicate unit of work in case the previous ACK corresponding the firstly received unit of work with the same SDR sequence number was dropped. 
     The SDR sequence number field  306  value being greater than the expected next sequence number value stored in the destination SDR resources indicates that the current unit of work is received ahead of its defined order, and thus, an intermediate unit of work is missing. A first option for responding to the indication that an intermediate unit of work is missing is for the destination SDR resources to silently drop the current unit of work and await the source SDR resources to retransmit the missing unit of work based, for example, on a timer expiring. 
     A second option for responding to the indication that an intermediate unit of work is missing is for the destination SDR resources to provide a NAK, which contains the expected next sequence number value in the SDR sequence number field of the protocol header of the NAK, indicating to the source SDR resources that an intermediate of work is missing. In one embodiment implementing the second option, the unit of work is dropped and the NAK indicates to the source SDR resources the sequence number of the missing intermediate unit of work. The source SDR resources respond to the NAK and retransmit all units of work having an assigned SDR sequence number value equal to or greater than the SDR sequence number value corresponding to the missing intermediate unit of work. 
     In another embodiment implementing the second option for responding to the indication that an intermediate unit of work is missing, the destination SDR resources verify other protocol header fields. If all other verification checks pass, the destination SDR resources temporarily store the current unit of work, while the middleware AIs perform a resynchronization operation to recover the missing intermediate unit of work. 
     Resynchronization is herein defined to be the process employed to synchronize the SDR associated with a given &lt;source AI, destination AI&gt; tuple. In other words, resynchronization aligns the SDR contents to determine what units of work have been posted for transmission, what units of work have been reliably completed, and what units of work need to be retransmitted. The resynchronization process is primarily controlled by the SDR sequence number values associated with a given SDR to retransmit and/or clean up the non-completed units of work. While resynchronization is performed strictly on an implementation dependent basis, the resynchronization process can also employ a barrier control message containing SDR state and control values, such as SDR sequence numbers and acknowledgment numbers, where the barrier control message is transmitted between the source and destination devices. 
     Error recovery for a given &lt;source AI, destination AI&gt; tuple typically involves performing a resynchronization event between the resources associated with the source AI and the destination AI. The resources associated with the source AI and the destination AI contain sufficient state information to recover all operations which have not been completed (i.e., neither an ACK nor a NAK has been received by the source Al to complete the unit of work transmission). The resource and the unit of work state information can be maintained at any location within the source and destination devices. 
     The example protocol header  300  illustrated in  FIG. 5  also includes source AI and destination AI identifier fields  308 . The source AI and destination identifier fields  308  are employed to perform completion events, identify the source Al to the destination AI for subsequent application-specific exchanges, and the like. 
     If the AIs are employing sender-based communication, example protocol header  300  contains additional protection fields  310 . Example protection fields  310  includes Hamlyn-style protection key, memory buffer address and offsets, and the like. Protection fields  310  are validated by the destination SDR resources before memory access is granted. 
     According to the example reliable communication protocol, each unit of work must be acknowledged using either an ACK, a NAK, or a communication services/fabric error notification. Acknowledgments are used to indicate whether a unit of work has reached the destination SDR resources and some form of action has been performed in response thereto. 
     In one embodiment, the acknowledgments (e.g., ACK, NAK, and communication services/fabric error notification) are implemented as stand-alone unit of work exchanges which are separate and well defined. In one embodiment, the acknowledgments are encapsulated in the unit of work within a protocol header. In one embodiment, the acknowledgments are formed as a component within a protocol header which is either transmitted separately or piggy-backed within another unit of work being transmitted in the other direction. 
     In one embodiment, ACKs are on a per unit of work basis. In this embodiment, a separate ACK is transmitted for each unit of work which is successfully received and processed by the destination SDR resources. In another embodiment, the ACKs are cumulative. In the cumulative ACK embodiment, for a given set of units of work, a single ACK is transmitted with the embedded SDR sequence number indicating that all units of work in the set of units of work up to and including the unit of work assigned the current SDR sequence number have been successfully received and processed by the destination SDR resources. An AI can have multiple units of work in-flight at any given time depending upon the underlying communication services/fabrics capabilities. The number of units of work that a given AI can have in-flight at a given time is not limited by the possible scope of the reliable datagram service according to the present invention, but can possibly be limited by specific implementation details of the SDRs and the underlying communication services/fabric capabilities. 
     In one embodiment, NAKs are generated on a per unit of work basis for each unit of work which is incorrectly received at the destination SDR resources. Example reasons for a unit of work to be incorrectly received at the destination SDR resources include cyclic redundancy check (CRC) error, protection violation, resource shortage, unrecognized destination AI, and the like. For each unit of work which is incorrectly received, a NAK is transmitted from the destination SDR resources and the NAK includes appropriate information to allow the source AI or the underlying communication services/fabric to determine the recovery action to perform in response to the NAK. If the NAK does not require a resynchronization event, the NAK serves as an acknowledgment for the unit of work to allow subsequent units of work flowing through the same destination SDR resources to be processed as through no error had occurred. 
     Acknowledgments (e.g., ACK, NAK, and communication services/fabric error notification) act as synchronization events between the source and destination ends of a SDR coupling the two devices of a &lt;source, destination&gt; device tuple to ensure that all units of work transmitted from the source device, independent of the AIs involved, are reliably delivered to the destination device. In one embodiment, acknowledgments also act as acknowledgments at the AI level, allowing a source AI to be assured that the units of work transmitted from the source AI are reliably delivered to a corresponding destination AI. In this embodiment, unit of work retirement is automatically processed based on ACKs. 
     Thus, there is a distinction between units of work delivered to a destination device and units of work delivered to a destination AI on the destination device. Delivery location and acknowledgement semantics determines what responding action the source AI should perform. For example, if a unit of work is delivered to the destination device but not to the destination AI, the source AI cannot assume that the unit of work has actually been consumed by the destination AI, and thus, the source AI must be careful as to whether subsequent actions should be performed. 
     An illustrative example is as follows, if a source AI is moving a disk block from a first device to a first destination AI on a second device and then transferring ownership of the disk block to a second destination AI on a third device, the source AI needs to be assured that the disk block was actually received by the first destination AI and was acted upon by the first destination AI before the source AI transfers ownership to the second destination AI. If the disk block was not actually received by the first destination AI or was not acted upon by the first destination AI before the source AI transfers ownership to the second destination AI, a subsequent failure within the first destination AI could result in the second destination AI disk block owner reading stale data. 
     Example Transmission Operations 
     An example transmission operation between source device A indicated at  202  and destination device C indicated at  206  for data processing system  200  is illustrated in diagram form in  FIG. 6 . As indicated in  FIG. 6 , SDR resources  212   a  at source device  202  include a queue  240  holding transmitted but not ACKed units of work and a queue  242  holding units of work not yet transmitted. At the time indicated in  FIG. 6 , queue  240  includes the following units of work: UW 1 ; UW 2 ; UW 3 ; UW 4 ; and UW 5 . The units of work held in queue  240  have been processed and transmitted by SDR resources  212   a  of source device  202 . At the time indicated in  FIG. 6 , queue  242  includes the following units of work: UW 6 ; UW 7 ; UW 8 ; and UW 9 . SDR resources  212   a  also store an expected next sequence number value  244  which is equal to 6 at the time indicated in  FIG. 6 . SDR resources  212   a  also store an ACK value  246 , which is equal to 0 at the time indicated in  FIG. 6 . 
     As illustrated in  FIG. 6 , destination device C indicated at  206  includes a queue  248  holding received units of work. The received units of work held in queue  248  at the time illustrated in  FIG. 6  include: UW 1 ; UW 2 ; and UW 3 . As indicated respectively at  252  and  254 , the units of work UW 4  and UW 5 , which have been transmitted from SDR resources  212   a , are on the communication services/fabric  218  at the time indicated in  FIG. 6 . At the time indicated in  FIG. 6 , SDR resources  212   b  include an expected next sequence number value  258  which is equal to 4 and an ACK value  260  which is equal to 3. SDR resources  212   b  have transmitted a stand alone ACK, indicated at  262 , for the received unit of work UW 1  and a cumulative ACK, indicated at  264 , for the received units of work UW 2  and UW 3 . Again, the decision to transmit ACKs as stand alone ACKs or cumulative ACKs is implementation dependent. In one embodiment, a piggy-back ACK on a unit of work flowing from destination device  206  to source device  202  can be employed to carry the ACK back to source device  202 . As units of work are transmitted, source device  202  and destination device  206  track what units of work have been acknowledged via ACK value  246  for device  202  and ACK value  260  for device  206 . As units of work are transmitted, devices  202  and  206  via SDRs  212   a  and  212   b  also track the expected next sequence numbers via expected next sequence number value  244  for device  202  and expected next sequence number value  258  for device  206  to ensure reliability is maintained. 
     Another example transmission operation between source device A indicated at  202  and destination device C indicated at to  206  for data processing system  200  is illustrated in diagram form in  FIG. 7 . In the transmission operation of  FIG. 7 , the state of SDR resources  212   a  of device  202  are substantially similar to the state of SDR resources  212   a  in the transmission operation of  FIG. 6 . Thus at the time indicated in  FIG. 7 , queue  240  of device  202  includes: UW 1 ; UW 2 ; UW 3 ; UW 4 ; and UW 5 . Queue  242  includes UW 6 ; UW 7 ; UW 8 ; and UW 9 . The expected next sequence number value  244  is equal to 6 and the ACK value  246  is equal to 0. In addition at the time indicated in  FIG. 7 , device  206  has received UW 1 , UW 2 , and UW 3  into queue  248 . Also similar to the transmission operation illustrated in  FIG. 6 , SDR resources  212   b  include the expected next sequence number value  258  equal to 4 and the ACK value  260  equal to 3. Additionally, UW 4  and UW 5  are on the communication services/fabric  218  as indicated at  252  and  254  respectively. 
     However, in the transmission operation of  FIG. 7 , SDR resources  212   b  of device  206  have issued a cumulative ACK for UW 1  and UW 2 , as indicated at  266 . In addition, SDR resources  212   b  of device  206  have issued a NAK for UW 3 , as indicated at  268 . In one embodiment, the NAK for UW 3  indicated at  268  also contains an error code to indicate the type of error detected to facilitate the resynchronization process. For example, if a CRC error is detected, UW 3  can be transparently retransmitted without involving the source AI. If, however, the detected error is that the destination AI is not operational, the source AI needs to be informed that UW 3  and any other units of work which target the destination AI have failed. Once the source AI has been informed that UW 3  and any other units of work which target the destination AI have failed, the source AI determines the necessary recovery technique to be used. For such a non-operational destination AI error, devices  202  and  206  increment the ACK value indicated respectively at  246  and  260  and the expected next sequence number value respectively indicated at  244  and  258 , because a resynchronization operation is not required and subsequent units of work in-flight or to be transmitted can be processed as though the error did not occur assuming that the units of work target different destination AIs. 
     Another example transmission operation between source device A indicated at  202  and destination device C indicated at  206  for data processing system  200  is illustrated in diagram form in  FIG. 8 . The example transmission operation of  FIG. 8  illustrates that the reliable datagram service according to the present invention guarantees strong ordering of the received units of work at destination device  206  when the units of work flow through the same SDR (e.g., SDR  212 ). 
     In the transmission operation of  FIG. 8 , the state of SDR resources  212   a  of device  202  are substantially similar to the state of SDR resources  212   a  in the transmission operations of  FIGS. 6 and 7 . Thus, at the time indicated in  FIG. 8 , queue  240  of device  202  includes: UW 1 ; UW 2 ; UW 3 ; UW 4 ; and UW 5 . Queue  242  includes UW 6 ; UW 7 ; UW 8 ; and UW 9 . The expected next sequence number value  244  is equal to 6 and the ACK value  246  is equal to 0. 
     However, in the example transmission operation of  FIG. 8 , the expected next sequence number value  258  of SDR resources  212   b  of destination device  206  is equal to 3 indicating that SDR resources  212   b  are expecting UW 3  as the next unit of work in the serial unit of work stream from SDR resources  212   a  of source device  202 . However, queue  248  of device  206  has received UW 1 , UW 2 , and UW 4 . In addition, as indicated at  270 , UW 3  is still on the communication services/fabric  218  behind the already received UW 4  and just ahead of UW 5 , indicated at  254 , in the serial unit of work stream from SDR resources  212   a  of source device  202 . Although the transmission operation of  FIG. 8  has UW 3  out of its defined order in the unit of work stream by one unit of work position to more clearly illustrate a strong ordering violation, typically a strong ordering violation occurs when a unit of work is completely missing from the unit of work stream. SDR resources  212   b  have issued a cumulative ACK for UW 1  and UW 2  as indicated at  266 . SDR resources  212   b  have also issued a NAK for UW 3 , as indicated at  272 , which indicates a sequence number violation (i.e., a strong ordering protocol violation). In this example, the SDR sequence number of the protocol header of UW 4  is equal to 4 which is larger than the expected next sequence number value  258 , which is equal to 3. This strong ordering protocol violation in this example transmission operation indicates that UW 3  is missing. Thus, SDRs  212   a  and  212   b  are resynchronized as the result of the NAK of UW 3  indicated at  272 . 
     Error Detection and Processing 
     Error detection and processing with the reliable datagram service according to the present invention is a function of the underlying communication services/fabric and the type of communication (e.g., sender-based or receiver-based communication) being employed. The underlying communication services/fabric and the type of communication being employed each provide error detection services which are generally independent of whether a given data processing system employs a reliable datagram service to communicate. Therefore, the following description is restricted to a description of the detection and processing of the type of errors which directly impact the reliable datagram service operation according to the present invention. 
     A first type of error which directly impacts the reliable datagram service operation is a protocol violation. A first type of protocol violation is a protection related violation. Examples of protection violations include: the unit of work protocol header containing invalid protection keys; invalid access right request (e.g., the request is to write to memory window but the destination AI has designated the memory window as read-only); memory address and bounds check violation; and the like. The protection errors are detected and a NAK is generated indicating the protection error so that the source AI can take appropriate corrective actions. The NAK in response to the protection error acts as a SDR acknowledgment and does not require a resynchronization event to be initialized. 
     A second type of protocol violation error is a sequence number error. A sequence number error occurs when the SDR sequence number field of the protocol header is either smaller or larger than the expected sequence number of the destination SDR resources. The SDR sequence number field value being less than the expected next sequence number value stored in the destination SDR resources indicates that the unit of work is a duplicate unit of work. The SDR sequence number field value being greater than the expected SDR sequence number value stored in the destination SDR resources indicates that the current unit of work is received ahead of its defined order, and thus, an intermediate unit of work corresponding to the expected next sequence number value is missing. 
     In one implementation, the sequence number check is actually a valid sequence number window check. In this implementation, sequence numbers are implemented using a fixed range (e.g., a 32-bit range yields 4 billion values). Within this range, a valid window is used to determine whether a unit of work is valid or not valid. This is a sliding window to account for the eventual sequence number roll-over. In one embodiment, sequence number check is implemented as representing half of the entire range (e.g., 2 billion if using a 32-bit sequence number). If the unit of work is within this range and less than what is expected, then it is a duplicate. If it is greater than the value, then either it is outside the window or it indicates that an intermediate unit of work was lost within the fabric. This will result in the unit of work being dropped, silently accepted but not completed, or a NAK can be generated indicating unit of work was dropped. Thus, in this implementation, the sequence number validation is a window validation check and the window is a sliding window. 
     The SDRs are resynchronized as a result of a sequence number error. In one embodiment, if the unit of work is determined to be a duplicate, the unit of work is silently dropped by the destination SDR resources. In one embodiment, the destination SDR resources drop the duplicate unit of work and provide an ACK indicating to the source SDR resources that the last unit of work was received to avoid the source SDR resources from again transmitting the duplicate unit of work in case the previous ACK corresponding the firstly received unit of work with the same SDR sequence number was dropped. 
     If the current unit of work is received ahead of its defined order indicating that an intermediate unit of work is missing as a result of the SDR sequence number field value being greater than the expected SDR sequence number, the destination SDR resources can take one of several actions. In one embodiment, the destination SDR resources silently drop the unit of work and await the source SDR resources to retransmit the missing unit of work based, for example, on a timer expiring. In one embodiment, the destination SDR resources generate a NAK in response to the indication that the unit of work is received ahead of its defined order and optionally drop the unit of work or temporarily store the unit of work into a received but unacknowledged queue. The NAK generated by the destination SDR resources informs the source SDR resources of the sequence number error and the expected next sequence number value. In one embodiment, the source SDR resources selectively retransmit unacknowledged units of work in response to the NAK indicating that a unit of work was received ahead of its defined order. In one embodiment, the source SDR resources retransmit all unacknowledged units of work in response to the NAK indicating that a unit of work was received ahead of its defined order. 
     Another type of error that directly impacts the reliable datagram service operation is the receipt of a corrupt unit of work. In this type of error, the unit of work or the attached protocol header is corrupted, such as by a CRC violation. In some situations, the destination device is not capable of trusting any portion of the unit of work when the receipt of a corrupt unit of work error occurs. In these situations, the destination device drops the unit of work and generates a NAK indicating the unit of work was corrupted. The next expected sequence number is included in the NAK so that the source device can determine which units of work are possibly lost and retransmit the unacknowledged units of work. The SDRs are resynchronized as a result of the receipt of a corrupt unit of work error. 
     Another type of error that directly impacts the reliable datagram service operation is a source AI or a destination AI abort error. No matter where the source AI or destination AI abort error occurs, the units of work which are in-flight either need to be flushed or completed so that all resources can be recovered. In one embodiment, if the source AI is aborted, the source device invalidates the unacknowledged units of work, employs a small control structure to account for all in-flight units of work so that the units of work can be completed even though the units of work are no longer valid, and shuts down communications. The source AI and destination AI abort error does not require a resynchronization event, and the flush operation is treated as a series of acknowledgements to insure all units of work on both sides of the &lt;source, destination&gt; device tuple have been acknowledged and all resources have been recovered. 
     Another type of error that directly impacts the reliable datagram service operation is the invalid destination AI error. The invalid destination AI error occurs when a source AI sends a unit of work to a destination AI which never was valid or is no longer valid. The destination device generates a NAK indicating the invalid DAI error for each unit of work targeting the invalid destination AI. The source device completes the units of work as normal and no resynchronization event is required with the invalid destination AI error. 
     Quality of Service 
     In one embodiment, quality of service (QoS) is implemented with the reliable datagram service according to the present invention by segregating the source AI traffic across a set of replicated SDRs. QoS relates to the scheduling of resources and AIs via service policies. QoS also relates to the employment of the service policies to effect the throughput and response times of a given AI unit of work stream. 
     Each SDR coupled between a &lt;source, destination&gt; tuple can be scheduled independently by the source device, the destination device, and the underlying communication services/fabric. This type of independent scheduling allows an application independent QoS policy to be implemented by middleware AIs. In one embodiment, each SDR is assigned a unique QoS. In one embodiment, SDRs are grouped into QoS levels where each QoS level is assigned a unique QoS. 
     Replicating SDRs creates the following generalized application benefits. First, AI communication resource contention is reduced because the communication resource contention can be spread across the multiple SDRs. Secondly, the number of AIs impacted by a given AI&#39;s behavior is reduced. As an illustrative example, strong ordering is preserved, in part, because a given unit of work transmission must be completed before subsequent unit of work transmissions can be acknowledged and completed. Therefore, if two source AIs are sharing the same SDR, the order that the source AIs issue requests is maintained by the SDR. 
     A third benefit to replicating SDRs is that when an error occurs, only the AIs employing the impacted SDR(s) are effected. Thus, all other AIs can continue to operate depending upon the error type. Transient errors, such as a CRC error, are recoverable. Hard errors, such as an error occurring because a physical link between AIs has failed, are recoverable if an alternative path physical link exists between the AIs and sufficient state information is available to successfully perform the replacement of the failed physical link with the alternative path physical link. 
     A portion of a data processing system having multiple SDRs established between device pairs according to the present invention is illustrated generally at  400  in  FIG. 9 . Data processing system  400  includes a reliable datagram service according to the present invention which has improved application performance and scalability because middleware AIs in data processing system  400  establish multiple SDRs between a device A indicated at  402  and a device B indicated at  404 . An example middleware AI for device  402  is indicated at  406 . An example middleware AI for device  404  is indicated at  408 . 
     A SDR  410  including SDR resources  410   a  at device  402  and SDR resources  410   b  at device  404  implements a reliable datagram service between device  402  and device  404 . A SDR  412  including SDR resources  412   a  at device  402  and SDR resources  412   b  at device  404  implements a reliable datagram service between device  402  and device  404 . A SDR  414  including SDR resources  414   a  at device  402  and SDR resources  414   b  at device  404  implements a reliable datagram service between device  402  and device  404 . A SDR  416  including SDR resources  416   a  at device  402  and SDR resources  416   b  at device  404  implements a reliable datagram service between device  402  and device  404 . The source and destination ends of each of the SDRs  410 ,  412 ,  414 , and  416  communicate via a communication services/fabric  418 . Device  402  and device  404  can each be a source or a destination device depending on the direction of communication. 
     Thus, device A indicated at  402  and device B indicated at  404  communicate together via reliable datagram service provided by four established SDRs  410 ,  412 ,  414  and  416 . The example configuration illustrated in  FIG. 9  is for illustrative purposes only and similar configurations can be established between an arbitrary number of devices. As illustrated in  FIG. 9 , a set of AIs are bound to each SDR restricting the impact any AI has on another AI to generally only occur if the AIs share the same SDR. 
     In the example embodiment illustrated in  FIG. 9 , device  402  includes an AI  420  coupled to SDR resources  410   a ; and AI  422  coupled to SDR resources  412   a  and SDR resources  414   a ; AIs  424 ,  426 ,  428 , and  430  each coupled to SDR resources  414   a ; and AIs  432 ,  434 ,  436 , and  438  each coupled to SDR resources  416   a . Device  404  includes an AI  440  coupled to SDR resources  410   b ; an AI  442  coupled to SDR resources  412   b  and SDR resources  414   b ; AIs  444 ,  446 ,  448 , and  450  each coupled to SDR resources  414   b ; and AIs  452 ,  454 ,  456 , and  458  each coupled to SDR resources  416   b.    
     SDR resources  410   a ,  412   a ,  414   a , and  416   a  are serviced based on the scheduling heuristics maintained at SDR schedule heuristics  460  in device  402 . Similarly, SDR resources  410   b ,  412   b ,  414   b , and  416   b  are serviced based on the scheduling heuristics maintained at SDR schedule heuristics  462  in device  404 . In this way, SDR schedule heuristics are used to adjust the scheduling rate to create different QoS for the AIs bound to the SDRs. 
     In the embodiment illustrated in  FIG. 9 , AI  420  and AI  440  are exclusively assigned to SDR  410 . When and if two AIs, such as AIs  420  and  440 , are exclusively assigned to one SDR is determined by: a middleware AI local to a device, such as middleware AI  406  of device  402  and middleware AI  408  of device  404 ; a middleware AI executing remotely and acting as a central manager, such as middleware AI  409 , which is accessed via the underlining communication services/fabric  418 ; and/or an application policy heuristics, such as stored at SDR schedule heuristics  460  of device  402  and SDR schedule heuristics  462  of device  404 . Normally, such a decision is based on the application or device service level objectives. 
     A given AI, such as AI  422  and AI  442 , can be bound to multiple SDRs if the given AI establishes multiple reliable datagram endpoints to communicate through. In such a case, each endpoint is serviced based on SDR scheduling heuristics, such as stored at SDR schedule heuristics  460  in device  402  and at SDR schedule heuristics  462  in device  404 . 
     Establishing multiple SDRs between any two devices in data processing system  400  over which multiple AIs may operate provides the following benefits. Since each SDR is generally mutually independent from other SDRs, the behavior of the AIs bound to one SDR does not generally impact the behavior of AIs bound to another SDR. For example, an AI which processes large units of work can be bound to a different SDR so that it does not impact the performance of an AI which processes small units of work. 
     Another benefit of establishing multiple SDRs between two devices in data processing system  400  is that errors which occur on one SDR generally do not impact the behavior of AIs bound to another SDR. In addition, error recovery may be shorter or simplified depending upon the number of SDRs between any two devices and the type of error detected (e.g., transient, protcol, communication services failure, and the like). 
     Another benefit of establishing multiple SDRs between two devices in data processing system  400  is that a middleware AI, such as middleware AI  406  of device  402 , may modify the SDR scheduling heuristics, such as SDR schedule heuristics  460  of device  402 , to transparently adjust the service rate a given AI receives relative to other AIs. For example, an AI which is primarily processing high priority units of work can be bound to a SDR which is scheduled ahead of SDRs which are processing lower priority units of work. This modification of the SDR scheduling heuristics permits the middleware AI to create QoS policies based on the service level objectives associated with a given AI. 
     Another benefit of establishing multiple SDRs between two devices in data processing system  400  is that within a given priority band, multiple AIs may be segregated across multiple SDRs, with the SDRs within the priority band being serviced using simple scheduling policies, such as round-robin or weighted round-robin. The objective of the priority band being serviced using the simple scheduling policies is to reduce the potential contention on a given SDR to further limit the number of AIs impacted by a given AI&#39;s behavior when the SDR is shared among multiple AIs. The priority band being serviced using these simple scheduling policies improves AI scalability and overall performance of the applications. However, any type of arbitrary scheduling policies can be created for QoS. 
     In the reliable datagram service according to the present invention, striping refers to the technique of transmitting units of work over multiple SDRs from one source AI to one destination AI. If strong ordering is required when transmitting units of work from a source AI to a destination AI, the source AI needs to transmit the units of work on one SDR. But if weak ordering is possible when transmitting units of work from a source AI to a destination AI, the source AI can transmit the units of work on multiple SDRs (i.e., the source AI can employ striping) with some priority scheme, but with the potential that strong ordering is not maintained. For example, most multi-media applications can be transmitted with striping so that resolution at the receiving end improves over time, but without the pixels arriving in a strongly ordered manner. 
     As to establishing multiple SDRs for a given device pair, the reliable datagram service according to the present invention provides no limit on the number of SDRs which can be established between the given device pair. There are, however, practical design considerations to limit the number of SDRs established between a given device pair. For example, each added SDR includes corresponding added physical resources. Moreover, additional resources are required to schedule the unit of work traffic between the given device pair across multiple SDRs and the scheduling becomes more complex as the number of SDRs are increased. In addition, the number of priority levels assigned to the multiple SDRs is preferably kept at a practical design number. Thus, the number of the SDRs to establish between a given device pair and the priority levels to be assigned to the multiple SDRs is limited only by implementation design considerations. 
     Advantages of Reliable Datagram Service 
     As discussed in the Background of the Invention section of the present specification, AIs employing a reliable connection service must create one dedicated resource set per destination AI. By contrast, AIs employing the reliable datagram service according to the present invention can re-use the same resource set per multiple destination AIs. Thus, the reliable datagram service according to the present invention reduces the number of resource sets to create and manage which accordingly reduces AI implementation cost and design complexity. In this way, the reliable datagram service according to the present invention provides for highly scalable data processing systems. 
     Even though the reliable datagram service according to the present invention provides for highly scalable data processing systems, the reliable datagram service provides reliable communication between AIs including guaranteed ordering of units of work transmitted between AIs. The reliable datagram service according to the present invention provides the reliability of the reliable connection service described in the Background of the Invention section of the present specification by guaranteeing that the unit of work transmission is reliable so that AIs employing the reliable datagram service can rely on the underlying communication services/fabric to correctly deliver the units of work or on error notification in the event of an unrecoverable error. In this way, the reliable datagram service according to the present invention permits an AI to effectively off-load unit of work delivery to the reliable datagram service which reduces AI development costs. 
     Moreover, in one embodiment of a data processing system according to the present invention, unreliable datagram service is simultaneously supported on a communication services/fabric which supports reliable datagram service. In one embodiment, unreliable datagram service is simultaneously supported along with reliable datagram service by tagging a unit of work as unreliable and avoiding acknowledgment/error processing actions. In one embodiment, an unreliable datagram service is simultaneously supported along with a reliable datagram service by treating all data as reliable, and as a result, never entering an application unit of work recovery algorithm. 
     Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the chemical, mechanical, electromechanical, electrical, and computer arts will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.