Abstract:
A method, computer program product, and apparatus for reconstructing sessions on a network are described. The method allows for monitoring of quality of service at an application level as well as for understanding application specific events. This allows the method to be used to generate service detail records for usage based on application type for use in billing. It also allows the qualitative and quantitative analysis of quality of service based on application specific parameters. For example, for voice over IP calls, application events like adding participants and removing participants can be detected and billed accordingly. Also, the quality of service can be measured. For example, the actual latency can be compared to a predetermined latency amount set by a provider for voice over IP calls. Additionally, service detail records can be generated based on application specific events instead of generic flows, so usage billing can be performed based on factors such as a price per minute per leg, etc, with application specific events generated each time a leg is added or dropped.

Description:
RELATED APPLICATIONS 
   This application relates to, claims the benefit of priority of, and incorporates by reference, U.S. Provisional Patent Application 60/141,351, entitled “Method and Apparatus for Session Reconstruction” filed 28 Jun. 1999, having inventor Limor Schweitzer. 
   This application relates to the following group of applications. Each application in the group relates to, and incorporates by reference, each other application in the group. The invention of each application is assigned to the assignee of this invention. The group of applications includes the following. 
   
     
       
             
             
             
             
             
           
         
             
                 
             
             
                 
                 
                 
                 
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               Method and 
               Limor 
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               Reconstruction 
             
             
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   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of network management. In particular, the invention relates to session reconstruction in a network environment. 
   2. Description of the Related Art 
   The Internet protocol (IP) that is widely used on the Internet does not provide a committed quality of service. Several protocols have been developed to compliment standard implementations of IP to provide varying degrees of support for committed quality of service networks. 
   One set of extensions is the Differentiated Services (diffserv) specified by RFC 2474 and RFC 2475, that provides for using portions of the IP header information to store information about the types of service (TOS). Another approach is the resource reservation protocol (RSVP) specified by RFCs 2205–2210. In some instances, where appropriate, the two can be used together to provide a committed quality of service over an IP network. 
   The provision of a committed quality of service network is distinct from the monitoring the network and billing for usage of the network. Existing network monitoring processes such as RMON2, and RMON, specified by RFC 2074 and RFC 2021 are designed to report statistics based on information available in the packet headers, e.g. source and destination address. With RMON2, this can be broken down on a per port basis. The granularity of the reports depends on the sampling of the RMON trace. The returned statistics are basic measures of number of bytes and number of packets. 
   Netflow(™), from Cisco Corporation, San Jose, Calif., adds to these abilities by providing measures based on the terms of service, e.g. diffserv style flag, and the IP port used. Similarly, Firewall-1(™) and Floodgate-1(™) from Check Point Software Technologies, Ramat Gan, Israel, offers a similar set of features to Netflow(™). Both Netflow(™) and Firewall- 1 (™)/Floodgate-1 (™) focus on reporting per flow statistics. 
   Previous techniques do not support quality of service related evaluation of network usage. Previous systems do not allow for reconstructing sessions, where each session may be comprised of multiple flows. Previous systems do not provide for application specific event monitoring. Previous systems to not handle large volumes of data received over different network devices well. Accordingly, what is needed is a session reconstruction system that supports measuring quality of service, reconstruction of sessions that include multiple flows, application specific event monitoring within flows, and distributed session reconstruction. 
   SUMMARY OF THE INVENTION 
   A method and apparatus for reconstructing sessions on a network is described. The method allows for monitoring of quality of service at an application level as well as for understanding application specific events. This allows the method to be used to generate service detail records for usage based on application type for use in billing. It also allows the qualitative and quantitative analysis of quality of service based on application specific parameters. For example, for web applications, quality of service can be measured by the time from requesting a link till the close of the session by delivery of the whole page. Similarly, for voice over IP calls, application events like adding participants and removing participants can be detected and billed accordingly. Also, the quality of service can be measured. For example, the actual latency can be compared to a predetermined latency amount set by a provider for voice over IP calls. Additionally, service detail records can be generated based on application specific events instead of generic flows, so usage billing can be performed based on factors such as a price per minute per leg, etc, with application specific events generated each time a leg is added or dropped. Further, because the periodicity of the output can be controlled on a per application basis, output for voice over IP calls can be generated more often than for other applications. Additionally, embodiments of the invention can reconstruct sessions that are flowing across multiple network devices. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  illustrates a system including one embodiment of the invention. 
       FIG. 2  illustrates the handling of packets that are not part of a recognized flow. 
       FIG. 3  illustrates the handling of packets that are part of a recognized flow. 
       FIG. 4  illustrates the relationship between information about a flow and information about a session. 
       FIG. 5  illustrates information generated by some embodiments of the invention for from session information. 
       FIG. 6  illustrates a situation in which distributed session reconstruction may be desirable. 
       FIG. 7  illustrates a system including one embodiment of the invention configured to support distributed session reconstruction. 
   

   DETAILED DESCRIPTION 
   A. System Overview 
   1. Terminology 
   The Internet protocol (IP) is a network layer protocol. The transmission control protocol (TCP) and the user datagram protocol (UDP) are two transport protocols used over IP networks. These transport mechanisms are in turn used by application layer protocols such as telnet, file transport protocol (ftp), hypertext transfer protocol (http), domain name service (DNS), simple mail transport protocol (SMTP), RealAudio(™), NetMeeting(™), etc. 
   One common encapsulation of IP packets is within IEEE 802.3 Ethernet frames. In such an embodiment, the payload, or data, portion of the packet includes an IP datagram comprising headers and a text part. The text part in turn includes the transport layer protocol such as TCP or UDP. The transport layer portion has an additional transport layer specific header and then a data portion. This data portion is in turn comprised of data specific to the application layer protocol. 
   Thus, a given packet sent over an IP network may have several sets of addresses, including: a set of medium access (MAC) layer address; a set of network layer address; and a set of transport layer address. Additionally, there may be application specific addresses. 
   Most routing and flow detection/management software is limited to looking at the header addresses: MAC addresses, IP addresses, and TCP/UDP ports. TCP/UDP ports can be used for multiple purposes; therefore, unless the application data itself is examined, it may not be possible to provide accurate application based reporting. For example, port 80 is typically used for hypertext transfer protocol (HTTP) access. However, there is nothing to prevent a program from using that port for other data, e.g. online games. In fact, switching a protocol like RealAudio(™), which sometimes exhibits poor behavior due to network congestion, to a port used by a well-known service such as DNS can provide huge speed improvements for an end user.
         Comparison of TCP/IP Model with OSI Model       

   This specification uses terminology from the TCP/IP Model to describe networks. However, a brief description of the OSI Model is appropriate. The OSI model, or Open Systems Interconnection Reference Model, is a seven-layer model comprising the following layers: physical (1); data link (2); network (3); transport (4); session (5); presentation (6); and application (7). 
   The TCP/IP Model terminology used in this specification can be mapped onto the OSI Model as follows: host-to-network (1/2); Internet Protocol (IP) (3); Transmission Control Protocol (TCP) and/or User Datagram Protocol (UDP) (4); and application layer (7). The TCP/IP model does not include an analogous set of abstractions for layers five and six of the OSI Model. The application layer in the TCP/IP model is comprised of higher-level protocols such as file transfer protocol (FTP), hypertext transfer protocol (HTTP), etc. 
   Some embodiments of the invention may be adapted to work with OSI Model networks and may include appropriate detectors for operating at the presentation and/or session layers. 
   2. Application Identification and Sessions 
   Because of the ability of applications to use TCP and UDP ports arbitrarily, it is not adequate to rely on header information to determine what application a packet is being used for. For example, RealAudio(™) packets could be sent over the ports normally used for DNS. 
   Limiting the review of packets to just headers would not allow applications that use multiple TCP/UDP flows for a single session to be tracked. Common examples of such applications include H.323 calls and ftp sessions. Thus, a voice over IP program can establish multiple TCP/UDP flows for a single call. Similarly, each file transferred during an ftp session can use a distinct TCP/UDP flow. 
   Another limitation of scanning headers alone occurs in committed quality of service networks where it is important to be able to monitor and charge for usage based on relevant events for an application. 
   Therefore, the term “session” refers to a group of related flows within a definite time bound relating to an end user experience, each of the flows may share one or more common packet header elements. Thus, for the ftp application, a session is comprised of the flows containing the commands as well as of the flows used for transferring files. For a voice over IP call, the control flows as well as all of the flows containing voice and/or video data would be part of a single session. 
   Additionally, sessions can hierarchically be comprised of other sessions. For example, the process of accessing a single web page may be comprised of multiple HTTP sessions. Thus a “WWW session” might be considered to comprise all web activity by a user in a definite time bound, e.g. times out after X minutes without further activity. A WWW session could be comprised of page sessions for each retrieved page. The page sessions in turn could be comprised of one or more HTTP sessions, e.g. one or more flows for retrieving an object using the HTTP protocol. 
   Continuing the example of the voice over IP call, a provider might provide guarantees about average latency to customers. For example, the provider might promise that the average latency would not exceed Z ms. If the entire voice over IP call is treated as a single session, the latency can be measured and the appropriate compensation can be given if the latency guarantee was not met. Further, because application specific events can be monitored, addition and removal of call legs can be tracked and appropriate service detail records generated. Also, different application protocols may have different usage billing requirements. For example, voice over IP calls for a prepaid calling card must be checked every minute to ensure that a user does not exceed the minutes available to them. 
   3. System Setup 
     FIG. 1  illustrates a system including one embodiment of the invention. This could be used in conjunction with a corporate Intranet to provide policy based session management and monitoring. A provider of voice could use this over IP telephony to meter and monitor usage and provide a committed quality of service. 
   This paragraph lists the elements of  FIG. 1  and describes their interconnections.  FIG. 1  includes the packet sources  100   a–e , a filter  102 , an analyzer  104 , a data collector  106 , a policy  114 . The analyzer  104  includes a flow manager  108 , an application recognizer  110 , and a session streamer  112 . The packet sources  100   a–e  are coupled in communication with the filter  102 . The filter  102  is coupled in communication with the analyzer  104 . The analyzer  104  is coupled in communication with the data collector  106 . The filter  102 , the analyzer  104 , and the data collector  106  are capable of accessing the policy  114 . 
   The following describes the uses of the elements of  FIG. 1 . The packet sources  100   a–e  could be network connections, local computers, network computers, the Internet, and/or some other type of packet source. The packet sources  100   a–e  are sources of packets such as IP packets, IPX packets, and/or some other type of packets. 
   In some embodiments the filter  102  is provided to filter out packets. In other embodiments, no filter  102  is used. The filter  102  can be set to remove local traffic from further analysis, e.g. packets not leaving the corporate Intranet, or packets not travelling over a particular backbone. Additionally, if multiple analyzers like the analyzer  104  are being used, then multiple filters like the filter  102  can be used to segment the analysis. For example, all voice over IP calls might be filtered out by one filter but be the only thing passed through by another. This allows for tremendous flexibility in providing distributed analysis and meaningful analysis. In some embodiments, a standard packet capture (pcap) language is used to define the filter, e.g. “tcp and port 80 or dst net 192.168.0.0 mask 255.255.0.0”, etc. 
   Only those packets that meet the tests of the filter  102  are passed to the analyzer  104 . In some embodiments, the filter  102  and the analyzer  104  are hosted on separate computers. For example, the filter  102  might have two Ethernet interfaces, one for receiving packets from the packet sources  100   a–e  and the other for sending matching packets to the analyzer  104 . 
   Packets are analyzed by the analyzer  104  to be assigned to flows and then to sessions. The analyzer  104  can gather statistics about flows and sessions for use by the data collector  106 . Each of the components of the analyzer  104  can be performed on a single computer and/or multiple computers to support distributed processing. 
   The policy  114  controls how the system operates. For example, the policy might specify the ability of certain users or groups to perform certain tasks. The policy might control how much bandwidth certain users or groups get. The policy might control how users or groups are billed for usage. The policy may also control how different application events are treated, e.g. for voice over IP request minute by minute service detail records, etc. Other options include controlling when sessions, flows and/or packets are dropped, the contents of output from the data collector  106 , what application specific headers and statistics are being collected, and/or other options. 
   For example, for HTTP, the time from click to first reply and time from click till last TCP thread finished might be recorded as well as the base uniform resource indicator (URI). In some embodiments, the policy can include a series of pcap language style expressions together with output selectors as shown by the example in Table 1. 
                                       TABLE 1                   Output           Out   In       Expression   Period   Action   . . .   Latency   Latency                   192.168.100/24 AND   Period = 60   Bill   . . .   Y   Y       (TIME &lt; 14:00:00 OR       TIME &gt; 22:00:00)       PORT &lt; 2000 AND   Period = 0   Log   . . .   Y   N       UDP       . . .                    
This allows a set of actions to be flexibly defined. A separate table could provide the information for the filter  102 . The policy  114  can also contain user and group based restrictions and evaluations.
 
B. Handling Unrecognized Flows
 
     FIG. 2  illustrates the handling of packets that are not part of a recognized flow. As users begin new activities, each flow is initially not recognized. For example, starting to access a web page.  FIG. 2  shows how unrecognized flows are handled according to some embodiments of the invention. 
   In this example, a filtered packet  200  is passed to the flow manager  108  within the analyzer  104  by the filter  102 . Because the flow manager  108  does not recognize the packet as belonging to an existing flow, it is added to a queue of unrecognized flows  202 A–B as unrecognized flow  202 C and the packets are placed in content  204 C. If additional packets for the flow arrive before the flow is recognized, they can be associated with the flow by adding the packet to the respective content  204 A–C. 
   The application recognizer  110  examines each of the flows in the queue and identifies whether the content of the flow matches a known application. This is based on the packet content itself. The application recognizer  110  can use the application tests  206  to perform matching. 
   In some embodiments, the application tests  206  include tests for CuSeeMe, http, ftp, RealAudio(™), post office protocol version 3 (POP3), SMTP, NetMeeting(™), Quicktime(™), H.323 calls, telnet, and/or other applications. The application tests for a particular application protocol describe how to identify a particular application protocol from the data content of packets. 
   In this example, three sessions  210 A–C have already been identified. If the application recognizer finds a matching application, the unrecognized flow  202 A will be assigned to a new session, session  210 D. The session streamer  112  is used to alert the flow manager  108  to new flows that are part of an existing session in some embodiments of the invention. Therefore, unrecognized flows will be assigned to new sessions while new flows for an existing session will be treated as recognized flows. 
   Some flows may not be recognized as belonging to any application. For example, if a new protocol is developed for streaming media, then none of the application tests  206  may be able to recognize the flow. In that case, some embodiments of the invention treat the unrecognized flow as a self-contained session after more than two kilobytes (KB) have been sent or if a predetermined amount of time passes without additional packets. 
   Because the application tests  206  are modular, additional tests can be added, modified, and/or removed easily. The tests can, if appropriately designed, detect specific application protocols, e.g. RealAudio(™) type Y encoding, etc. 
   C. Handling Existing Flows 
     FIG. 3  illustrates the handling of packets that are part of a recognized flow once a session is underway (For example, ongoing packets in a voice over IP call). Additionally, the session streamer  112  can provide information to the flow manager  108  to allow new flows for an existing session to be recognized without the application recognizer  110  being used.  FIG. 3  shows how recognized flows are handled according to some embodiments of the invention. 
   Filtered packets  300  flow into the flow manager  108 . Because the packet belongs to a recognized flow, e.g. the recognized flow  3202 A, it is associated with the respective content, e.g. the content  304 A. 
   The session streamer  112  uses the application streamers  306  to detect application specific events, e.g. add leg, etc., and assign the content to respective sessions. The application streamers  306  are similar to the application tests  206 . However, the application streamers  306  contain tests for matching additional packets from the same application session. 
   The session streamer  112  in conjunction with the application streamers  306  may also be able to detect the request for additional channels or ports and provide that information to the flow manager  108 . Thus, new flows for an existing session will not be treated as unrecognized flows, but rather will be recognized and handled by the session streamer  112 . For example, the application streamers  306  might include NetMeeting(™) specific streamers for detecting add leg and drop leg events and providing the addressing information to the flow manager  108 . The policy  114  can assign significance and actions relative to certain application events identified by the application streamers  306 . 
   The session streamer  112  assigns the packets from the flows to the respective sessions based on the results of the application streamers  306 . Here, the recognized flow  302 A and the content  304 A is matched with the session  210 B. 
   D. Statistics Generation 
     FIG. 4  illustrates the relationship between information about a flow and information about a session.  FIG. 4  includes three flows  400 A–C with respective packet time-stamps  402 A–C. Each flow  400 A–C is associated with a corresponding session  210 A–D. Here, the flows  400 A–B are both associated with session  210 D while flow  400 C is associated with session  210 A. The packet time-stamps  402 A–C are used to generate the statistics  404 A–D corresponding to each of the sessions. 
   If reporting is performed solely on a per flow basis, it does not capture the overall performance of the session. Nor does it capture the performance from an application specific fashion. For example, an H.323 call is may be comprised of at three or more flows. For example, for a call from John to Jane, there might be two flows for audio and a third flow for control. Per flow monitoring alone could suggest that one flow for the call, e.g. John to Jane, is meeting the committed quality of service. But, nothing would connect that information with the fact that the other flow, Jane to John, is not. 
   Further, if there is billing taking place, then it is important that the billing be aggregated on a per session basis with meaningful service detail billing. For voice over IP telephony, that might be a charge per minute per leg. For HTTP, that might be a charge per megabyte. A service detail record can include a billing identifier, e.g. user name, calling card number, phone number, and/or some other identifier. The service detail record also can include the usage within the interval covered by the service detail record. For example, a service detail record for a voice over IP call might include the phone number and the usage, e.g. “650/555-1212, 5 legs, 3007 sec ttl”, etc. For an Internet backbone provider, service detail records generated might be at the ISP level and measured in megabytes in a fixed interval, e.g. “isp1 300.7 MB”. 
   Some statistics computed by embodiments of the invention include: flow-level statistics, start time, end time, time since last output, number of packets, number of bytes, average time between packets, moving average, latency, throughput, jitter, and/or other statistics. The jitter is the standard deviation of the latency and throughput. When appropriate, the statistics can be further subdivided between input and output information. Latency is an application specific computation in some embodiments of the invention. For example, with TCP packets, latency can be determined by looking at the time between sequential acknowledgements. In contrast, for a real-time protocol, the latency might be calculated as the difference between the end of communication in a control flow and the start of communication in a data flow. 
   E. Output Generation 
     FIG. 5  illustrates information generated by some embodiments of the invention for from session information. As  FIG. 5  shows, the generated statistics, e.g. the statistics  404 A–D, for sessions can be provided to the data collector  106 . The policy  114  can be used to define the output of the data collector  116 . 
   Outputs include usage reports  500  that describe application usage in application specific terms, e.g. 700 minutes of voice over IP calls, maximum of 10 simultaneous calls, etc. Service detail records  502  are another output of the data collector  106 . These could be output at application specific intervals, six seconds for voice over IP, every hour for web usage, etc. The service detail records  502  can be used for billing purposes and also to limit access if the paid for usage is exceeded. 
   For example if a user purchases twenty minutes of voice over IP calls, when she/he reaches that limit, systems monitoring the service detail records  502  can terminate the call, etc. 
   Another output can include quality of service reports  504 . These may specify, on an application level, the performance for the session, as appropriate, this can be presented in application specific terms. For example, if a voice over IP call should have no more than a Z ms latency to avoid echo, the report might specify how many calls exceeded that latency and by how much. 
   Another output might include router commands  506  to control a router, e.g. to limit further usage or re-prioritize usage of bandwidth relative to performance and committed quality of service. For example, if RealAudio(™) sessions consume too much bandwidth relative to the priority set in the policy  114 , the router commands  506  could block the routing of RealAudio(™), or reduce its priority further to allow higher priority sessions to proceed at the committed quality of service. 
   In some embodiments, aspects of the different reports are combined. For example, the service detail report  502  might include the quality of service of a voice over IP call and if the committed quality of service is not delivered, the usage charge might be waived. 
   F. Distributed Session Reconstruction 
   1. Description of the Problem 
   The foregoing discussion has focused on a setting in which all packets are visible to a single session reconstruction system. However, in many configurations it may not be possible, or desirable, to provide all packet data to a single point. 
     FIG. 6  illustrates a situation in which distributed session reconstruction may be desirable. A client computer  600  and a host computer  602  are coupled in communication over a packet switched network including two routers, the router  604  and the router  606 . Two examples will be considered, one involving the file transfer protocol and the other involving asymmetric routing. 
   In the first example, the flows from a simple FTP session are shown as a dotted path between the client computer  600  and the host computer  602 . Here, a flow  608  is the control flow for the FTP session and is established across the router  606 . Meanwhile, the flow  610  is a transfer flow in the FTP session and is established across the router  604 . 
   Assume, for the sake of argument, that the packets flowing through the router  606  are sent to a session reconstruction system of the type described above as the packet source  100   a , but that packets flowing through the router  604  are provided to a different session reconstruction system. The session reconstruction system monitoring the packets from the router  606  will be able to detect the FTP session and the control flow  608 . The other session reconstruction system, monitoring the packets from the router  604 , may be able to detect the transfer flow  610 , but may not be able to identify the protocol or the appropriate application session. 
   In the next example, the flow  608  and the flow  610  represent two halves of a single communication. This occurs when the traffic from the client computer  600  to the host computer  602  traverse a different set of network devices than packets sent in the other direction, e.g. asymmetric routing. Again, as in the example above, if the two routers are supplying their traffic data to different session reconstruction systems, it may not be possible to monitor even a single flow from one session reconstruction system. 
   In these instances, neither session reconstruction system would be able to provide a complete description of the session. As networks become more heavily meshed and redundant, situations like the one depicted in  FIG. 6  are likely to occur more frequently. 
   2. Solution to the Distributed Case 
   Solutions to this problem could include providing all of the raw data to a single session reconstruction system. This approach does not scale well. As the number of packet sources increases, the bandwidth and computation power required for session reconstruction goes up. For example, consider a session reconstruction system coupled to an ATM switch, that system might be working at capacity. Adding packets from three or four additional ATM switches for analysis may not be a viable option computationally or in terms of bandwidth. 
   Accordingly, some embodiments of the invention operate in a semi-hierarchical fashion. This allows session reconstruction to be distributed over several systems of the type described above.  FIG. 7  illustrates a system including one embodiment of the invention configured to support distributed session reconstruction.  FIG. 7  does not show the policy  114 , however such a policy can control the filters  102   a–c , analyzers  104   a–d  and data collector  106   a . Additionally, the policy  114  can have different rules for different modules, if appropriate. For example, filter  102   a  and filter  102   b  might have different rules in the policy  114  to filter out local traffic. 
   As seen in  FIG. 7 , the basic configuration of each session reconstruction system is according to the manner described above. A packet source (e.g. the packet source  100   f ) flows into a filter (e.g. the filter  102   a ) and then to an analyzer (e.g. the analyzer  104   a ). The difference lies in the disposition of results from the initial analysis—including unrecognized flows. In the system of  FIG. 7 , results from the analyzers  104   a–d  can be passed to other analyzers (e.g. the analyzer  104   d ). This approach can be further nested with the analyzer  104   d  coupled to other analyzers higher in the hierarchy, not shown. Additionally, when appropriate, data can be transferred directly from an analyzer (e.g. the analyzer  104   c ) to a data collector (e.g. the data collector  106   a ) as shown by the dotted line in  FIG. 7 . This would be appropriate if a session has been fully re-constructed by an analyzer (e.g. the analyzer  104   c ). 
   The messages between analyzer levels can now be considered in more detail. There are two basic cases to consider: when the analyzer (e.g. the analyzer  104   a ) has complete session information and when the analyzer does not have complete session information. 
   When a given analyzer (e.g. the analyzer  104   a ) can determine complete session information, the session data, together with statistics, can be sent to a higher level analyzer (e.g. the analyzer  104   d ) or directly to the data collector (e.g. the data collector  106   a ). 
   When it is not possible for a given analyzer (e.g. the analyzer  104   a ) to determine complete session information, there are a number of approaches for providing information to higher level analyzers in the hierarchy. Three primary approaches will be considered: packet forwarding, hints together with forwarded packets, hints together with summary of packets. It is possible to use combinations of these approaches in a single system. Additionally, other approaches can be used.
         a. Packet Forwarding       

   In many respects, this is the simplest of the three approaches; however, it is also inefficient from a bandwidth perspective. In this approach, packets that can not be constructed into sessions are forwarded as raw packet data—together with time stamps—to higher level analyzers. 
   At the higher level analyzer, the raw packet data from the different lower level analyzers can be integrated and considered. In many instances, e.g. the example of  FIG. 6 , this will allow for session reconstruction. Thus, if the analyzer  104   a  was handling the data from the router  606  and the analyzer  104   b  was handling the data from the router  604 , both analyzers would forward the raw packet data to the analyzer  104   d  which would now be able to recognize the entire FTP session.
         b. Hints Plus Packet Forwarding       

   This approach reduces some of the computational complexity of the first approach, e.g. the need for the higher level analyzers to reprocess all packet data. In this approach, hints are extracted by the lower level session analyzer and provided to higher level analyzers. Additionally, as in the first approach, the raw packet data is forwarded—together with time stamps—to higher level analyzers. 
   Turning again to the example of  FIG. 6 . If the analyzer  104   a  is handling the data from the router  606 , the application streamer might detect the request to establish a file transfer over certain ports within the control flow  608 . This information can be provided to the higher level organizer as a hint. Additional, a second hint could be provided that identifies the packets being forwarded by the analyzer  104   a  as an FTP control flow. The hints reduce the amount of processing required by the higher level analyzer. 
   For example, the higher level analyzer could use the second hint to identify the forwarded packets as belonging to an FTP session and use the first hint to prime the application streamer to recognize the transfer flow packets when they are forwarded from another analyzer.
         c. Hints Plus Summary of Packets       

   This approach is the most complex, but also the most bandwidth efficient. In this approach, as above, hints are generated at lower level analyzers. Additionally, incomplete data is aggregated into a summary whenever possible. The summary can include information from packet headers as well as attributes and metrics. Examples of data that would be included in summaries are: source address, destination address, source port, destination port, terms of service (TOS), protocol quality of service (QOS), number of packets, number of bytes, latency, etc. 
   Using this approach, the control flow  608  could be reduced to a summary together with a hint for recognizing the transfer flow  610 . The transfer flow  610  might be reduced to a summary by another analyzer and the hint would allow a higher level analyzer to group the summary information about the transfer flow  610  as part of a single FTP session with the control flow  608  summary information. 
   In some instances, it may not be possible to summarize a group of packets. In that case, it may be necessary to forward the raw packet data. In some embodiments, small flows and/or packets are sent rather than generating a summary. This is efficient because the cost of sending a summary about a small packet or an extremely short flow may exceed the cost of re-transmitting the small packet or extremely short flow. 
   The different approaches are each backward compatible with the previous approaches. Thus, two hint methods can accept data in packet forwarding format, e.g. without hints. Similarly, the hint plus summary method can also accept hints together with forward packets, e.g. without summaries. 
   3. Additional Configurations 
   Some additional configurations used by some embodiments of the invention should be discussed. If desired, the filter, e.g. the filter  102 , can be omitted. Similarly, when arranging a hierarchy of distributed session reconstruction modules, different streams can traverse different components. For example, in  FIG. 7 , the analyzer  104   c  could be omitted in preference for allowing the packet source  100   h  to be analyzed first by the analyzer  104   d . These variant arrangements can reduce hardware and software costs associated with using embodiments of the invention while also increasing the flexibility with which embodiments of the invention can be deployed. 
   G. Alternative Embodiments 
   In some embodiments, the filter  102 , the analyzer  104 , the flow manager  108 , the application recognizer  110 , the session streamer  112 , the data collector  106 , the policy  114 , the application tests  206  and the application streamers  306  are included in hardware, software, and/or a combination of hardware and software. 
   In some embodiments, the filter  102 , the analyzer  104 , the flow manager  108 , the application recognizer  110 , the session streamer  112 , the data collector  106 , the policy  114 , the application tests  206  and the application streamers  306  are included as one or more computer usable media such as CD-ROMs, floppy disks, and/or other media. 
   Some embodiments of the invention are included in an electromagnetic wave form. The electromagnetic wave form comprises information such as the flow manager  108 , the application recognizer  110 , the session streamer  112 , the application tests  206 , and/or the application streamers  306 . For example, the application streamers  306  might include a database of application streamer data accessed over a network by the session streamer  112 . 
   H. Conclusion 
   The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to limit the invention to the precise forms disclosed. Many modifications and equivalent arrangements will be apparent.