Patent Publication Number: US-2007124485-A1

Title: Computer system implementing quality of service policy

Description:
BACKGROUND  
      In computer systems, particularly enterprise networks, managing “quality of service” can be important. In this context, “quality of service” relates to how well users&#39; expectations of the performance of the system are met. In a networked computer system, a user&#39;s perception, and therefore quality of service, is heavily influenced by the latency with which different types of information is transmitted over the network. For example, a network may deliver datagrams with latencies that vary between microseconds and several hundreds of milliseconds, depending on network loading. Such latencies will be adequate for many applications. However, for some applications, such as VoIP and other applications that provide an interactive experience to a user, such variations in latency will be noticeable, causing the user to perceive a low quality of service.  
      Poor quality of service frequently occurs when too many datagrams need to be transmitted through a network “bottleneck.” Datagrams are queued at the bottleneck, creating latency. In an enterprise, a bottleneck may occur at a connection to the Internet or other similar gateway at the edges of the enterprise network. Bottlenecks can also occur within an enterprise network, such as at a trunk line connecting zones within the network.  
      To improve quality of service, network components can be designed to process datagrams with different priorities. Datagrams for which high latency creates a poor user experience may be assigned a higher priority. Giving priority to these datagrams keeps latency for those datagrams low, even at a bottleneck, and increases the overall quality of service. Alternatively, datagrams for which high latency has relatively little effect on user perception of the quality of the network may be transmitted with lower priority.  
      The ability to manage quality of service has been available in some computer systems. For example, codes, called ToS or DSCP codes, may be inserted in datagram headers. A router or other network element constructed to recognize ToS or DSCP codes will typically maintain different queues and assign a datagram to a queue based on the DSCP or ToS code. Datagrams in a queue associated with a higher priority code will be given higher priority in selecting the next datagram to process, which reduces latency for higher priority datagrams relative to lower priority datagrams.  
      Additionally, drivers that manage the transmission of datagrams over a network can be equipped to “throttle” datagram transmission in relation to a setting provided with a datagram. When a throttle setting is applied to a datagram, the driver may buffer the datagram before transmitting it over the network to keep the rate of transmission below a rate specified by the throttle setting.  
      Applications have been developed that specify DSCP or throttle settings as a way to control latency of datagrams and increase quality of service.  
     SUMMARY OF INVENTION  
      In one aspect, the invention relates to a computer system in which a quality of service policy may be implemented. The computer system has features that are useful in many settings, including computer systems that are part of enterprise networks.  
      In one aspect, the invention relates to a client computer to implement a quality of service policy defined by policy information. As the client computer establishes connections, portions of the policy information applicable to each connection are identified and cached. When a datagram is transmitted using a connection, the cached portion of the policy information associated with that connection is used to select quality of service parameters applicable to the datagram. Such a client computer may apply a quality of service policy without introducing substantial computational delay.  
      In another aspect, the invention relates to a software architecture, which may be incorporated into a computer system in many ways, such as by being a part of the operating system of a client computer. The architecture has computer-executable components including a driver, a quality of service component and a network stack. The network stack has an interface through which it may receive a message for transmission over a network. In response to receiving a message through the stack interface, the network stack may receive a quality of service parameter from the quality of service component. The stack may then supply the datagram and the quality of service parameter to the driver. Such a software architecture allows a quality of service policy to be implemented in a computer system without the need to modify applications to implement the quality of service policy.  
      In another aspect, the invention relates to storing policy information in connection with time information. Policy information may be stored along with a first time value. Policy information may be derived from this information associated with a specific connection. The information for a connection may be stored with a second time value. Storage of information in conjunction with time values allows for datagram-by-datagram checks that policy information is up-to-date, without introducing a high computations burden.  
      The foregoing is a non-limiting summary of the invention, which is defined by the attached claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings:  
       FIG. 1  is a sketch of an enterprise network using a quality of service policy;  
       FIG. 2  is a software block diagram of a prior art client computer;  
       FIG. 3  is a software block diagram of a client computer adapted to implement a quality of service policy;  
       FIGS. 4A, 4B , and  4 C are sketches illustrating data structures that may be used in an embodiment of a system implementing a quality of service policy; and  
       FIG. 5  is a flow chart showing a method of operation of a computer system to implement a quality of service policy. 
    
    
     DETAILED DESCRIPTION  
      A particular difficulty in implementing a quality of service policy is associating transmission characteristics with datagrams. Requiring applications to specify the transmission characteristics for messages they generate can lead to a system that is difficult to implement and maintain.  
      As used herein, a quality of service policy is information that defines conditions under which one or more datagram transmission characteristics are set or altered. The transmission characteristics described herein influence perceived network latency, but any suitable characteristics may be specified as a portion of a quality of service policy. Also, the quality of service policy may be specified in any suitable forms and may exist in multiple forms. For example, the policy may be defined in human-readable form and may also be embodied as a computer-readable data structure.  
      We have recognized that implementation of a quality of service policy in a computer system, particularly an enterprise network, can be simplified without undesirably increasing the computational burden on the computer system. The software components needed to implement the quality of service policy can be included in the operating system running on client computers, allowing widespread use of the quality of service policy throughout a network without the need to modify applications to implement the policy.  
      By separating the software that sets transmission characteristics of datagrams in accordance with a quality of service policy from the applications that initiate transmissions of those datagrams, policy information may be downloaded into clients throughout the network. Being able to download policy information allows the quality of service policies to be updated as network configuration or traffic conditions change. In one embodiment, the quality of service policy information is downloaded using a policy management service already available within the network, further simplifying the widespread use of a quality of service policy throughout an enterprise network.  
      However, applying a quality of service policy on to each datagram or message as it is transmitted runs the risk of unreasonably increasing the computational burden on each client and therefore average latency of all network traffic. Embodiments of client software implementing a quality of service policy are described in which transmission characteristics can be selected for each datagram in accordance with a quality of service policy without unreasonable computational overhead or delay.  
       FIG. 1  illustrates an embodiment of a computer system that employs a quality of service policy. In the example of  FIG. 1 , the computer system is enterprise network  100 , which is configured to employ a quality of service policy. Enterprise network  100  includes clients  110 A and  110 B. A client may be any device connected to the network that injects datagrams into the network for transmission to other networked devices reachable through the network.  
      In this context, the term “datagram” refers generally to a unit of information formatted for transmission over a network. Sometimes, datagrams are called “packets” or “messages” or may be given other names in connection with networks using specific protocols. Use of the general term “datagram” herein is intended to signify that the inventive concepts described herein are broadly applicable to communications regardless of specific format or network protocol.  
      In this example, clients  110 A and  10 B are desktop workstations. However, any suitable computing device, whether now known or hereafter developed, may serve as a client. In this example, two clients  110 A and  110 B are shown for simplicity. An actual enterprise network may have many clients.  
      A network traditionally includes one or more switching devices that interconnect devices in the network. A switching device directs datagrams transmitted over the network to the appropriate device based on header information in the datagram. There are many types of switching devices, such as devices commonly called “switches,” “hubs” and “routers.” The specific network architecture and hardware is not a limitation of the invention. Accordingly, router  116  is used as an example of a switching device.  
       FIG. 1  shows a simplified enterprise network  100 . As used herein, the term “enterprise” does not connote a network or company of any specific size, but refers generally to a group of computing devices for which a person or entity may provide control or other operational information. In the embodiment described, policy information defining operation of multiple clients is provided by a network administrator and stored on database  114 .  
      In  FIG. 1 , enterprise network  100  is connected through gateway  118  to a broader network. In this example, the broader network may be the Internet  130  and the connection may be made through a trunk line  120 . However, it is not necessary for enterprise network  100  be connected to a public network. For example, gateway  118  may connect enterprise network to other network zones managed by the same enterprise that manages enterprise network  100 . The connection through gateway  118  allows client computers  110 A and  110 B within enterprise network  100  to communicate with other devices outside of enterprise network  100 . For example, client computers  110 A and  110 B may communicate with a server  150  through router  140  that is connected to Internet  130 .  
       FIG. 1  provides examples of where “bottlenecks” may occur for enterprise network  100 . Switching devices, such as routers  116  and  140  and gateway  118  may create bottlenecks. Likewise, trunk line  120  could create bottlenecks. Accordingly, it may be desirable to use a quality of service policy within enterprise network  100  to reduce the impact of these bottlenecks on user perception.  
      To facilitate the implementation of policies within enterprise network  100 , enterprise network  100  also includes server  112 . In this example, server  112  acts as a source of policy information stored in database  114 . Clients such as  110 A and  110 B connect to server  112  to download policy information from database  114 . The policy information stored in database  114  may represent any type of policy information, such as configuration settings for communication software, formatting options to apply to documents or other policies.  
      Policy servers, such as server  112 , are known in the art. For example, Microsoft, Inc. provides an enterprise application called “Active Directory” that includes a server configured to provide policy information to multiple clients in an enterprise network.  
      Enterprise network  100  may be conveniently implemented with an application, such as Active Directory, that would otherwise exist in an enterprise network. However, enterprise network  100  differs from conventional enterprise networks in that the policy information in database  114  relates to a quality of service policy instead of or in addition to other policy information as known in the art. In addition, devices that receive the policy information are adapted to apply the quality of service policy information.  
      In embodiments described herein, policy information is in the form of a set of rules. However, any suitable format for representing policy information may be used. Each rule may specify one or more conditions and one or more transmission characteristic to be used when the conditions are met. In the embodiments described herein, the transmission characteristics specified as part of a quality of service policy rule may be used to influence the relative latency of a datagram transmitted with those characteristics. The transmission characteristics may be codes, such as DSCP or ToS codes, that are inserted in message headers. Routers or other network devices may use these codes to prioritize datagrams for transmission or forwarding.  
      Transmission characteristics included in a policy may additionally or alternatively specify a maximum rate at which data is to be injected onto the network. A client or other device that transmits datagrams using such a characteristic may ensure the maximum rate is not exceeded by “throttling” transmission of datagrams. A device may “throttle” transmissions by buffering datagrams to be transmitted to leave sufficient time between datagrams that the maximum rate is not exceeded. However, any desired approach to throttling may be used.  
      Regardless of the specific form in which the policy information is stored in database  114 , once downloaded to a client computer, the client computer can implement the policy as applicable to datagrams transmitted by that client. As a client initiates transmission of a datagram, it determines which portion of the policy is applicable to that datagram and appropriately applies it. In some embodiments, a quality of service policy is implemented by software within the operating system of the client computer.  
      Implementing a quality of service policy from within the operating system software provides advantages over the conventional approach of requiring each application that generates datagrams to specify applicable transmission characteristics. One advantage is that no changes are required to applications in order for the application to implement a quality of service policy. Therefore, a network administrator can quickly and easily set or modify a quality of service policy as a network or network usage changes.  
      Even though the quality of service policy is implemented differently than in the prior art, conventional components for implementing a quality of service policy may nonetheless be employed.  FIG. 2  provides an example of a software architecture such as may be found in client computers as known in the art.  FIG. 2  shows components of an operating system as is conventionally found on a client computer.  
      In  FIG. 2 , network stack  214  has an interface  212  through which application  210  may pass a message or other information for transmission over a network. Application  210  may represent a word processor, a web browser or other application program executing on a client computer. Traditionally, a client computer includes multiple applications, but only one is shown for simplicity. Stack  214  may be a network stack as is found in an operating system, such as the WINDOWS® operating system.  
      Network stack  214  formats information passed through interface  212  as appropriate for transmission as one or more datagrams. Information specifying the datagrams is then passed to driver  216 . Driver  216  controls network interface card (NIC)  218  to transmit the datagrams over the network.  
      In the embodiment of  FIG. 2 , interface  212  may be adapted to receive, in addition to information specifying a message to be transmitted, information specifying transmission characteristics. In this example, the transmission characteristics specified are a throttle rate and a DSCP value. This information is passed through network stack  214  to driver  216 . Driver  216  inserts the DSCP value into the header portion of each datagram it sends to transmit any portion of the message initiated by application  210 .  
      Driver  216  also receives the throttle value provided by application  210  in conjunction with a datagram to be transmitted. Driver  216  uses the throttle value to limit the rate at which datagrams are sent. If applications within a client computer are coded to generate quality of service parameters in association with messages initiated by the applications, the datagrams encapsulating those messages will be transmitted with characteristics that implement the policy.  
      As shown in  FIG. 2 , a client computer may include the client portion of a policy service used within an enterprise network. Client policy service  250  may be a software module that controls the client to receive policy information from a policy server, such as policy server  112  ( FIG. 1 ). However, in the prior art embodiment in  FIG. 2 , policy service  250  did not receive quality of service policy information because such information needed to be encoded as part of an application  210 .  
       FIG. 3  shows a software architecture according to an embodiment of the invention. In the embodiment of  FIG. 3 , application  310  need not be specially configured to implement a quality of service policy. Application  310  simply passes information for transmission through interface  312  to network stack  314 . Operating system components apply a quality of service policy to the message. An advantage of this architecture is that messages from any application may be subject to a quality of service policy even if the application is not customized to provide quality of service information in conjunction with messages it generates.  
       FIG. 3  shows a software architecture for client software that includes a quality of service (QoS) policy component  320 . QoS policy component  320  provides transmission characteristics for datagrams as they are being processed by stack  314 . In the illustrated embodiment, stack  314  calls an interface to QoS policy component  320  each time stack  314  processes a message to be transmitted. QoS policy component  320  returns values defining transmission characteristics. The transmission characteristics may be the same as controlled in conventional systems. In this example, the transmission characteristics are a throttle rate and a DSCP value, but any suitable transmission characteristics may be specified to implement a quality of service policy.  
      QoS policy component  320  may receive policy information from a centralized location, such as policy server  112  ( FIG. 1 ). Client policy service component  250  is constructed to generally perform functions needed to obtain policy information but is customized to obtain specific types of policy information through the use of client side extensions. Here, QoS client side extension  322  is configured to provide client policy service component  250  with information needed to identify quality of service policy information on server  112 . In addition, QoS client side extension  322  provides control information necessary for client side policy service component  250  to store policy information in the appropriate locations.  
      In the embodiment illustrated, QoS client side extension  322  identifies to client policy service component  250  that QoS policy registry  324  is the destination for downloaded quality of service policy of information. In addition, QoS client side extension  322  provides control information to QoS inspection module  328 . Such control information may provide notification from QoS client side extension  322  to QoS inspection module ( 328 ) that a policy update has occurred or otherwise coordinate action of the components.  
      In the embodiment illustrated, client policy service component  250  obtains policy information from server  112 . To ensure each client contains up to date policy information, client policy service component  250  may periodically pull information form the server. However, any suitable timing and method of obtaining information from server  112  may be used.  
      As one example of another method of obtaining policy information, it is not necessary that all clients within an enterprise network obtain quality of service policy information from the same server or from a single server. Each network domain may have a domain server, which may also serve as a policy server. Further, even if domain servers are provided as a centralized repository of policy information, an enterprise network may additionally include other policy servers that provide policies for the entire network.  
      As another example of a further source of policy information,  FIG. 3  shows that each client may have a client policy registry  326 . Client policy registry  326  may be a data structure containing policy information specific to the client containing client policy registry  326 . Though the invention facilitates centralization of policy information, policy information also or additionally may be placed in client policy registry  326  on a client computer in any desired way. For example, it may be placed by a user operating the client computer, by an application when it is installed on that client computer or by a network administrator.  
       FIG. 3  shows that client policy information is stored in a separate data structure from policy information downloaded from server  112 . It is not necessary that the information be stored in a separate location or in a separate data structure. In the described embodiment, policy information from different sources is stored in a way that allows specific policies to be associated with a source so that, if a client receives inconsistent policies from different sources, it can reconcile the inconsistencies based in part on the source of the policies.  
      If a client obtains policy information from multiple sources, the client may reconcile the policy information to determine which policy to apply in the event of a conflict. In the described embodiment, policy information from multiple sources is reconciled according to the network hierarchy. For example, client policies, as the lowest level of the hierarchy, are applied only if not inconsistent with policies at any other level. At the other extreme, network policies are at the highest level of the hierarchy and may be applied in all instances.  
      In the embodiment illustrated, policies are specified as rules containing conditions. If the conditions of two rules are determined to both apply to the same transmission, then the highest priority rule is applied to that transmission. However, any suitable method of reconciling different policies may be employed.  
      To avoid the computational overhead of applying a QoS policy on a message-by-message or datagram-by-datagram basis, QoS policy component  320  may store policy information in a manner that allows fast identification of appropriate transmission characteristics for each transmission.  
      In some embodiments, fast identification of appropriate transmission characteristics is achieved by storing policy information associated with each connection formed by a client. As each connection is formed or modified, a portion of the total policy information potentially applicable to that connection is identified and cached. In the embodiment of  FIG. 3 , upon establishment of a connection, QoS inspection module  328  selects the policy information from QoS policy registry  324  and client policy registry  326  that is potentially applicable to the connection and caches it in a form that can be readily applied as each datagram is sent.  
      In this context, “connection” refers generally to predetermined information about some or all of a communication path between a client and another device, or to predetermined information about some or all of a communication endpoint on a client. The format of a connection may vary depending on the communication protocol used. For example, devices communicating using TCP protocol establish connections that are sometimes called sessions. A session involves communication from one application component in a client computer to another application component in a remote device. If the same application component in the client wants to communicate with a different application in the remote device, a different session is established. For a TCP connection, establishing a connection may allow QoS inspection module  328  to identify all aspects of a policy applicable to that connection.  
      On the other hand, devices communicating with a UDP protocol establish communication endpoints that describe only portions of the communication path from one application to another. When a datagram is sent using an endpoint, the datagram includes header information that identifies the application that originated the datagram and the application that is the destination for that datagram. Full information about the path is not available until the datagram is generated, but the information associated with a UDP “connection” nonetheless may allow portions of the policy to be identified as irrelevant to the connection.  
      The amount of information about the transmission path specified when a connection is created—or conversely the amount of information that must be specified about the transmission path when a datagram is transmitted—may dictate the amount of information about quality of service policies that needs to be stored in order to apply the policies to datagrams as they are transmitted through the connection. For a TCP connection, the full path is specified when the connection is established, which may allow all of the policy information to be evaluated to select transmission characteristics in accordance with the policy.  
      For a UDP connection, aspects of the policy that depend only on the source application or user name for the datagram can be evaluated. But, any aspects of the policy information that depend on the specific source or destination device, or destination applications, can not be evaluated until the datagram is generated. Thus, the portion and format of the policy information that is cached for a connection may depend on the protocol used for that connection.  
       FIGS. 4A-4C  illustrate how policy information may be cached to reduce the computational burden when a datagram-by-datagram application of the policy information is performed.  FIG. 4A  illustrates a data structure contained within QoS policy registry  324 . In this example, QoS policy registry contains all of the policy information applicable to a client. Where a separate client policy registry, such as client policy registry  326 , is used, the policy information in that registry may also be used in determining a portion of the policy information applicable to a connection.  
      In the embodiment illustrated, each policy is represented as a rule. Each rule is stored as a record in the data structure. Records  410 A,  410 B . . .  410 N are illustrated in  FIG. 4A . Each of the records  410 A,  410 B . . .  410 N is shown to contain multiple fields. Taking record  410 A as illustrative, the record contains a condition field  412 . Condition field  412  contains information defining the conditions under which the rule applies. The specific conditions for each rule depend on the quality of service policy for the applicable network. However, the conditions may specify characteristics of a datagram, such as the source device, the destination device, the source application or the destination device. In this example, any number of conditions or combinations of conditions may be specified and condition field  412  may contain any desired number of subfields to store the required information.  
      Each of the records also contains fields defining transmission characteristics that are to be applied when the rule is applicable. In the example of  FIG. 4A , two transmission characteristics are specified. Accordingly, each record contains two transmission characteristic fields defining transmission characteristics to be used for datagrams satisfying the conditions specified in the condition field. Again taking record  410 A as illustrative, a throttle field  414  and a DSCP field  416  are shown to specify transmission characteristics. These fields store a throttle value that may be provided to control driver  216  when a datagram is transmitted and a DSCP code that may be inserted into a header for a datagram.  
      The number of fields and type of data stored in each field is for illustration only. In clients equipped to control transmission characteristics other than a throttle rate and a DSCP code, further fields may be included to specify transmission characteristics. Alternatively, fewer fields may be included for clients that have less ability to control transmission characteristics.  
       FIG. 4A  shows QoS policy registry  324  also includes an field  430 . Field  430  stores a value indicative of the epoch or time at which QoS policy registry  324  was last updated. As described above, information in QoS policy registry  324  is periodically retrieved by policy service  250 . As will be described below, the time stored in field  430  is used to ensure that current information available is used in setting transmission characteristics for a datagram.  
      Regardless of the form in which policy information is stored, upon establishment of a connection, a portion of the policy information is cached.  FIG. 4B  shows an exemplary data structure  440  containing a cache of connection information. In this example, data structure  440  represents data associated with UDP connections.  
      Data structure  440  contains a plurality of records, of which records  442 A,  442 B and  442 C are illustrated. Each of the records corresponds to a connection and contains fields providing data that allows a quality of service policy to be applied to each datagram transmitted through the connection.  
      Taking record  442 A as illustrative, the record contains a field  444  identifying the connection. The data stored in field  444  may be in the format used by applications sending messages to identify the connection through which the message is to be transmitted. Storing data in this form reduces the computation required to select an appropriate one of the records  442 A,  442 B and  442 C to use when applying a quality of service policy to each datagram. However, any suitable format may be used for information stored in connection field  444 .  
      Record  442 A also includes a flag field  446 . For a UDP connection, the policy information stored in QoS policy registry  324  may make any number of rules applicable to a connection. In the illustrated embodiment, flag  446  provides a convenient way to identify the number of rules potentially applicable to the connection described by a record and may be used by a component applying the record to a datagram. In the illustrated embodiment, flag field  446  contains one of three values to indicate whether the record contains multiple rules applicable to a connection, a single rule applicable to a connection or no rules applicable to a connection.  
      Record  442 A illustrates the structure of a record containing multiple rules. Record  442 B illustrates the structure of a record containing no rules. Record  442 C illustrates the structure of a record containing a single rule. Regardless of the number of rules, each rule may be stored in the same format.  
      For example, record  442 A contains fields  448 A . . .  448 M, each storing information about one rule. Information about a rule could be stored in any convenient fashion. For example, fields such as  448 A . . .  448 M could contain pointers to a record in QoS policy registry  324 . However, in the described embodiment, each rule is stored by copying the record in QoS policy registry  324  corresponding to the rule.  
       FIG. 4B  shows that data structure  440  includes an field  450 . Field  450  stores a value indicating the time at which data structure  440  was updated. In some embodiments, the value in field  450  is set by copying the value in field  430  when data structure  440  is created or updated. When data structure  440  is used to compute a transmission characteristic for a connection, it is possible to quickly determine whether data structure  440  was formed using the most up to date policy information by comparing the value in field  450  to the value in field  430 . If the value in field  430  indicates a later time than the value in field  450 , data structure  440  is potentially out of date and may be recomputed for the policy information in QoS data structure  324 .  
       FIG. 4C  provides an example of a data structure  460  that may be used to store a portion of the policy information associated with TCP connections. As with data structure  440 , each connection is represented by a record in the data structure, of which records  462 A,  462 B and  462 C are shown. Taking record  462 A as illustrative, each record contains a field  464 A identifying the connection. In the present example, once a connection is specified using the TCP protocol, all information needed to apply all the policy rules is available. Accordingly, storing a portion of the policy information involves storing only the transmission characteristics dictated by the policy information. Accordingly, record  462 A contains a data field  464 B storing the transmission characteristics associated with a connection. The transmission characteristics stored in field  464 B may be stored in a suitable format. In this example, field  464 B may store data in the form stored in fields  414  and  416 .  
       FIG. 4C  shows that data structure  460  also includes an field  470  storing a time value. As with field  450 , field  470  is used to identify when the information in data structure  460  is out of date.  
      Here, data structures  440  and  460  are shown as separate data structures. Any suitable partitioning of the data may be used, including storing information about multiple types of connections in a single data structure. If information about multiple types of connections is stored in a single data structure, a field may be included for each record in the data structure indicating the type of connection, which would then indicate the number and types of fields contained in the record. Alternatively, identifying the connection may, in some instances, provide sufficient information to identify the type of connection.  
      Turning now to  FIG. 5 , a method of operating a client computer to implement a quality of service policy is shown. In an enterprise network, each client computer may operate according to a process as illustrated in  FIG. 5 .  
      The process of  FIG. 5  may be performed by software running on a client computer and may be performed by the operating system software of a client computer having the architecture as illustrated in  FIG. 3  and using data structures as illustrated in  FIGS. 4A  . . .  4 C. However, the process blocks shown in  FIG. 5  represent functions that can be readily implemented in many ways by one of skill in the art and any suitable implementation may be used.  
      The process of  FIG. 5  includes two sub processes. Sub process  520  occurs when a connection is established. Sub process  540  occurs when a datagram is transmitted. These processes are shown in  FIG. 5  as sequential because a connection is established before a datagram is transmitted through that connection. However, these sub processes could each be implemented as separate processes of a multi-process device.  
      The overall process begins at block  510  where the software waits for a connection. In a client, a connection is generally established by a call from an application program to components within the operating system. In some embodiments, the process of  FIG. 5  is initiated by the operating system component that is called to establish a connection.  
      When a connection is established, sub process  520  is performed to store a portion of the overall policy information that could be applicable to the connection. As described above, policy information may be available to a client from multiple sources. In some instances, a quality of service policy may be specified by thousands of rules. Processing at block  522  selects a subset of these rules. The selected rules are the ones that will always be applicable to a datagram transmitted through the connection or rules that may be applicable to such datagrams, depending on parameters associated with the datagram when it is generated.  
      Once the potentially applicable rules have been identified, processing continues to decision block  524  where the process flow splits based on whether the connection being established is a TCP connection. In the embodiment described, policy rules are specified based on conditions that can be evaluated at the time a TCP connection is established. Accordingly, if a TCP connection is specified, the transmission characteristics for that connection may be computed at block  526  by applying the rules selected at block  522 . The transmission characteristics may then be stored for the connection. That information may, for example, be stored in a data structure such as data structure  460  ( FIG. 4C ), including a value identifying when the rules used to compute the cached information were last updated.  
      Alternatively, if the connection to be established is not a TCP connection, processing proceeds for decision block  524  to block  530 . At block  530 , the potentially applicable rules selected at block  522  are cached at block  530 . The rules may be cached in a data structure such as data structure  440  ( FIG. 4B ), including a value identifying when the rules used to compute the cached information were last updated.  
      The process then continues to block  532  until a datagram for transmission is identified. When transmission of a datagram is initiated, the sub process  540  is performed. In some embodiments, sub process  540  may be initiated when stack  314  places a call to QoS inspection module, requesting transmission characteristics for a datagram.  
      As an initial step in sub process  540 , a comparison is made between the values stored for a particular cache, such as a value  450  or  470  stored in data structure  440  or data structure  460 , respectively.  
      At decision block  542  the process branches based on whether the comparison of values indicate the cached information is out of date. If so, processing returns to sub process  520  where the cached information is first updated before it is applied. Once it is determined that the cached information is up to date, processing proceeds to decision block  544 .  
      The process branches at decision block  544  based on the type of connection through which a datagram will be transmitted. If the connection is not a TCP connection, the process branches to block  548 . At block  548 , applicable transmission characteristics are computed using the rules cached at block  530 . Alternatively, if the connection is a TCP connection, processing proceeds to block  546  where the transmission characteristics cached at block  528  for the connection are retrieved.  
      Regardless of how the transmission characteristics are determined, processing the proceeds to block  550  where the datagram is transmitted with those characteristics. When the process of  FIG. 5  is performed on a client computer having a software architecture as pictured in  FIG. 3 , the transmission is made by returning to stack  314  values representative of the transmission characteristics. Stack  314  then provides those values to driver  216  for transmission. However, any suitable method of transmitting a message with the specified characteristics may be used.  
      Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art.  
      For example, the embodiment of  FIG. 1  illustrates communication within a network, but application of the invention is not limited to any specific network or network configuration. For example, though  FIG. 1  shows policy information stored in database  114  connected through a server  112  to clients within enterprise network  100 , no specific storage architecture for centralized policy information is required. Centralized policy information may be provided from a router or other network device or may be provided from one client to another or may be individually loaded into each client computer. Policy information may alternatively be provided from locations outside of network  100 . Further, embodiments of the inventions may be constructed without the use of centralized policy information at all.  
      As another example, quality of service policies have been described that specify transmission characteristics to influence the latency of datagrams. The concepts described herein are applicable to policy information that influences any transmission characteristic of datagrams. Further, the concepts described herein are not limited to controlling datagram characteristics and may be applied to implement a policy that influences user perception of network operation by controlling any other operational characteristic.  
      Also,  FIGS. 4A  . . .  4 C illustrate data structures useful for TCP and UDP connections. Though TCP and UDP are the most common protocols in use, a system may be constructed to work with other protocols. Similar data structures can be used in connection with other protocols.  
      Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.  
      The above-described embodiments of the present invention can be implemented in any of numerous ways. For example, the embodiments may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers.  
      Also, the various methods or processes outlined herein may be coded as software that is executable on one or more processors that employ any one of a variety of operating systems or platforms. Additionally, such software may be written using any of a number of suitable programming languages and/or conventional programming or scripting tools, and also may be compiled as executable machine language code or intermediate code that is executed on a framework or virtual machine.  
      In this respect, the invention may be embodied as a computer readable medium (or multiple computer readable media) (e.g., a computer memory, one or more floppy discs, compact discs, optical discs, magnetic tapes, etc.) encoded with one or more programs that, when executed on one or more computers or other processors, perform methods that implement the various embodiments of the invention discussed above. The computer readable medium or media can be transportable, such that the program or programs stored thereon can be loaded onto one or more different computers or other processors to implement various aspects of the present invention as discussed above.  
      The terms “program” or “software” are used herein in a generic sense to refer to any type of computer code or set of computer-executable instructions that can be employed to program a computer or other processor to implement various aspects of the present invention as discussed above. Additionally, it should be appreciated that according to one aspect of this embodiment, one or more computer programs that when executed perform methods of the present invention need not reside on a single computer or processor, but may be distributed in a modular fashion amongst a number of different computers or processors to implement various aspects of the present invention.  
      Computer-executable instructions may be in many forms, such as program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Typically the functionality of the program modules may be combined or distributed as desired in various embodiments.  
      Various aspects of the present invention may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments.  
      Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.  
      Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.