Abstract:
A device that passively monitors arriving and departing data packets on one or more networks, correlates arriving data packets with departing data packets, and calculates a latency estimate based on the confidence of the correlation. The device detects and copies data packets arriving at a network device and the data packets departing from the same network device. A timestamp is stored for each arriving or departing data packet. Latency across a network device can be determined based on the timestamps for correlating data packets. Additionally, latency across a network device per protocol layer can also be calculated. Varying levels of confidence of a latency estimation depend on the operation necessarily performed on the data packet by the network device and the protocol level at which correlation between the arriving and departing data packets can be achieved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to methods for measuring the performance of devices on a network, and in particular relates to passively calculating the time required for data packets to traverse a network device in a network.  
           [0003]    2. Related Art  
           [0004]    Comparison of network appliances is often characterized in terms of bandwidth and latency. For example, the performance of certain network appliances such as hubs, switches, routers, firewalls, and servers is often rated by the number of bytes per second the device can process. This type of bandwidth measurement is well defined and is conventionally easy to obtain.  
           [0005]    On the other hand, latency measurements require a certain amount of interpretation and are therefore conventionally difficult to obtain without intrusive probes. This is particularly underscored when attempting to determine the amount of delay, in microseconds per byte, for data packets traveling across routers, firewalls, and servers.  
           [0006]    [0006]FIG. 1 illustrates a conventional sniffer system as is typically used to measure the latency of a network device. In this system, data packets are sent from network  10  to network  20  through a unit under test (“UUT”)  30 . A sniffer  40  is placed between network  10  and UUT  30  in order to measure the latency caused by UUT  30  for data packets traveling between network  10  and network  20 . For example, UUT  30  may be a router or a bridge.  
           [0007]    Data may travel from network  10  to network  20  in discrete units called data packets. For example, data packet DP 1  may travel from network  10  to network  20  through UUT  30 . In transit between network  10  and network  20 , the sniffer  40  detects the transmission of DP 1  and generates and stores a timestamp for DP 1 , relative to the internal clock associated with the sniffer  40 .  
           [0008]    The DP 1  packet is then processed through UUT  30  and forwarded on toward network  20 , which returns data packet DP 2  in response. The response data packet DP 2  is processed back through UUT  30  and forwarded on to network  10 . The sniffer  40  detects the transmission of DP 2  and generates and stores a timestamp for DP 2 , relative to the internal clock associated with the sniffer  40 .  
           [0009]    The sniffer  40  subsequently compares the stored DP 1  timestamp (the time DP 1  was detected) with the stored DP 2  timestamp (the time DP 2  was detected) to determine a latency. A drawback of this conventional sniffer system is that the system does not measure the latency produced solely by UUT  30  as outgoing data packet DP 1  is processed and returned as incoming data packet DP 2 . An additional drawback of this conventional system is that the latency time for an entire network is measured rather than the latency time the UUT  30 .  
           [0010]    [0010]FIG. 2 illustrates a conventional two tap system for actively measuring the latency of a network device. Although this system employs two points of reference, which advantageously allows the system to isolate UUT  30 , the system is not passive. In operation, tap  50  creates a test data packet DP 3  and sends the test data packet DP 3  over the network to tap  60 . Due to the physical location of taps  50  and  60  relative to UUT  30 , the test packet DP 3  must travel through UUT  30 . Although this system isolates UUT  30 , it artificially increases the number data packets traveling on the network. Therefore, the latency measurement may not be accurate due to the artificially increased network traffic, introduced by the conventional two tap system.  
           [0011]    An additional drawback of the conventional two tap system is that it requires two discrete units to measure the latency for a network appliance. The first unit, tap  50 , is required to create the test data packet DP 3 , and the second unit, tap  60 , is required to retrieve the test data packet DP 3 . This two discrete unit configuration is problematic in certain test scenarios, for example firewall performance testing, because the firewall may or may not allow the single test data packet DP 3  to pass through.  
           [0012]    Furthermore, even when test data packet DP 3  is allowed to pass through unimpeded, there is little assurance that the test packet DP 3  received the same handling and processing as native network data packets. For example, firewalls that were configured to allow test data packets DP 3  to pass through may also be configured to give these same test data packets DP 3  a higher processing priority. Thus, the integrity of the latency measurement may be called into question for the conventional two tap system.  
           [0013]    Therefore, what is needed is a method and apparatus that overcomes these significant problems found in the conventional systems as described above.  
         SUMMARY OF THE INVENTION  
         [0014]    A device that passively monitors arriving and departing data packets on one or more networks, correlates the arriving data packets with the departing data packets, and calculates latency estimates based on the confidence of the correlation. The device detects and copies the data packets arriving at a network device and the data packets departing from the same network device, also called a unit under test (“UUT”). Latency across the UUT can be determined with varying levels of confidence, depending on the operation necessarily performed on the data packet by the UUT and the protocol level at which correlation between the arriving and departing data packets can be achieved.  
           [0015]    As data packets are detected and copied by the device, a high resolution timestamp is stored for each data packet prior to queuing the data packet for correlation. Data packets may be correlated at different protocol layers. The lowest correlator level can match data packets at the network specific frame level and can report a latency measurement as accurate as the timestamp resolution on the monitoring device. The highest correlator levels can match data streams that span more than one data packet.  
           [0016]    At the higher protocol levels, when data streams are correlated, a ceiling function may be applied to represent the timestamp of a particular data packet that has arrived out of sequence. For example, in the case of reconstructing a TCP stream, the timestamp for a particular data packet in the TCP stream will be the timestamp for the last data packet received that completes the data stream up to the particular data packet. In other words, if data packet  100  arrives before data packet  99 , the timestamp for data packet  99  will be applied to data packet  100 .  
           [0017]    Correlation of data packets and data streams can be achieved across multiple protocols. At the lowest level, the network specific frame correlator will detect the latency of network devices such as a switch or a repeater. At the highest level, the application correlator will detect the latency of network devices such as a World Wide Web (“Web”) server or a file transfer protocol (“FTP”) server. Any type of correlation may be applied to data packets, subject to external factors such as the amount of storage available to the monitoring device, and the percentage of network traffic visible to the monitoring device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The details of the present invention, both as to its structure and operation, may be gleaned in part by study of the accompanying drawings, in which like reference numerals refer to like parts, and in which:  
         [0019]    [0019]FIG. 1 is a block diagram illustrating a conventional sniffer system for passively measuring the latency of a network device;  
         [0020]    [0020]FIG. 2 is a block diagram illustrating a conventional two tap system for actively measuring the latency of a network device;  
         [0021]    [0021]FIG. 3 is a block diagram depicting an overview of a latency measurement device for passively calculating the latency of a network appliance according to an embodiment of the invention;  
         [0022]    [0022]FIG. 3A is a block diagram illustrating an overview of a latency measurement device for passively calculating the latency of a network device according to an embodiment of the present invention;  
         [0023]    [0023]FIG. 4 is a flow diagram illustrating an example data packet flow in a correlation process according to an embodiment of the invention;  
         [0024]    [0024]FIG. 5 is a flowchart illustrating a process for intercepting pre-processed data packets on a network according to an embodiment of the invention;  
         [0025]    [0025]FIG. 6 is a flowchart showing a conventional method for a network appliance to process a data packet received from a network;  
         [0026]    [0026]FIG. 7 is a flowchart illustrating a process for intercepting processed data packets on a network according to an embodiment of the invention;  
         [0027]    [0027]FIG. 8 is a flowchart illustrating a process for calculating a latency for a network appliance based on the timestamps of matching data packets according to an embodiment of the invention;  
         [0028]    [0028]FIG. 9 is a flowchart illustrating a process for correlating and matching data packets according to an embodiment of the invention;  
         [0029]    [0029]FIG. 10 is a flow diagram showing an exemplary data packet in transmission between two networks and through a latency measurement device according to an embodiment of the invention;  
         [0030]    [0030]FIG. 11 is a block diagram illustrating a conventional protocol layering principle widely used in TCP/IP networking environments;  
         [0031]    [0031]FIG. 12 is a flow diagram illustrating a conventional technique for demultiplexing incoming data packets based on a protocol type found in the data packet header;  
         [0032]    [0032]FIG. 13 is a flow diagram illustrating a conventional technique for demultiplexing incoming data packets based on a type found in the IP datagram header;  
         [0033]    [0033]FIG. 14 is a flow diagram illustrating a conventional technique for demultiplexing incoming data packets based on a type found in the TCP packet header; and  
         [0034]    [0034]FIG. 15 is block diagram illustrating an exemplary computer system in which elements of the present invention may be implemented according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    Certain embodiments as disclosed herein provide for a method and apparatus for passively calculating the latency of a network device. This is accomplished by detecting data packets that are arriving to and departing from the network device and correlating those packets to determine the latency introduced by the device. For example, one embodiment as disclosed herein allows for correlation of data packets ranging from network specific frames to application specific data streams.  
         [0036]    After reading this description it will become apparent to one skilled in the art how to implement the invention in various alternative embodiments and alternative applications. However, although various embodiments of the present invention will be described herein, it is understood that these embodiments are presented by way of example only, and not limitation. As such, this detailed description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention as set forth in the appended claims.  
         [0037]    [0037]FIG. 3 is a block diagram illustrating an overview of a latency measurement device for passively calculating the latency of a network device according to an embodiment of the present invention. Data packets flow, according to the diagram, from network  10  toward network  20 . Due to the physical layout of the system, data packets that traverse the unit under test (“UUT”)  30  when traveling between network  10  and network  20  also pass through tap  80  and tap  80 A. The latency measurement device (“LMD”)  100  detects the data packets that traverse UUT  30  through network taps  80  and  80 A.  
         [0038]    Taps  80  and  80 A may be any type of network connection as is well known in the art. Once tap  80  or  80 A detects a data packet traveling on the network, the tap may copy the data packet and forward the copy of the data packet to LMD  100 . The taps  80  and  80 A may be situated such that each data packing arriving at or departing from UUT  30  is detected. In one embodiment, UUT  30  is connected to two separate networks. Therefore, tap  80  may be connected to the first network while tap  80 A may be connected to the second network. Such an arrangement may allow the network taps  80  and  80 A to detect and copy each data packet arriving at or departing from UUT  30 .  
         [0039]    In an alternative embodiment, UUT  30  may be connected to a plurality of networks. In such an embodiment, LMD  100  may employ as many network taps as required to detect each data packet arriving at or departing from UUT  30  on any of the plurality of networks. Furthermore, the direction of flow for the data packets can be any of the possible directions available on the network. For example, data packets may flow from network  10  toward network  20 , as illustrated in FIG. 3. However, data packets may also flow from network  20  toward network  10 .  
         [0040]    Additionally, in one embodiment UUT  30  may be connected to only one network. FIG. 3A is a block diagram illustrating an overview of a latency measurement device for passively calculating the latency of a network device according to an embodiment of the present invention. UUT  30  may be a file server or some other sort of network appliance, such as a Web server. For example, requests for Web documents may be transmitted to Web server UUT  30  from network  10 . The corresponding Web documents may then be sent back to network  10  a response from UUT  30 .  
         [0041]    Tap  80  may be advantageously situated between network  10  and UUT  30  such that each incoming request and each outgoing response is detected by tap  80 . Furthermore, each data packet detected by tap  80  may be copied and forwarded to LMD  100 . LMD  100  may then correlate each incoming data packet with its corresponding outgoing data packet in order to calculate a latency estimation for UUT  30 .  
         [0042]    [0042]FIG. 4 is a flow diagram illustrating an example data packet flow in a correlation process according to an embodiment of the invention. Data packets flow in the system between network  10  and network  20 . LMD  100  is configured such that any data packets arriving at UUT  30  or departing from UUT  30  are detected by either tap  80  or tap  80 A. Those packets detected by tap  80  or tap  80 A are copied by the tap and the data packet copy is forwarded to LMD  100  for correlation.  
         [0043]    Data packets forwarded to LMD  100  may be received by a network interface card (“NIC”). In one embodiment, LMD  100  has an NIC that corresponds to each network tap. In FIG. 4, NIC  92 A corresponds to tap  80  and NIC  92 B corresponds to tap  80 A. Once an NIC receives a data packet from the network tap, the data packet may be processed up the protocol stack while also being queued for correlation at each layer.  
         [0044]    For example, a communication originating from the application layer may be received by NIC  92 A and multiplexed in each protocol layer up to the application layer. Additionally, at each level in LMD  100 , the data packet may be queued for correlation. For example, a network specific frame of the communication originating from the application layer may be received by network layer  94 A and passed to frame correlator  90 A for queuing and subsequent correlation. If the network specific frame is part of a larger communication, the frame may be combined with other frames into a datagram and passed up to Internet layer  96 A.  
         [0045]    Similarly, Internet layer  96 A may pass the datagram to IP correlator  90 B for queuing and subsequent correlation. If the datagram is also part of a larger communication, the datagram may be combined with other datagrams into a packet and passed up to TCP layer  98 A. TCP layer  98 A may pass the packet to TCP correlator  90 C for queuing and subsequent correlation. If the packet is part of a larger communication, the packet may be combined with other packets into a message and passed up to Application layer  99 A. Finally, Application layer  99 A may pass the packet to Application correlator  90 D for queuing and subsequent correlation.  
         [0046]    A similar process may take place for packets received by NIC  92 B. For example, frames received by Network layer  94 B may be multiplexed into datagrams and passed to IP layer  96 B after being queued for correlation. Datagrams received by IP layer  96 B may be multiplexed into packets and passed to TCP layer  98 B after being queued for correlation. Packets received by TCP layer  98 B may be multiplexed into messages and passed to Application layer  99 B after being queued for correlation. And messages received by Application layer  99 B may be queued for correlation. Advantageously, this parallel processing and redundant correlation of data packets may increase the robustness of correlation at higher levels of the protocol layer.  
         [0047]    In one embodiment, correlation of data packets that are deposited in the particular queues takes place by matching the unique characteristics of corresponding data packets. For example, correlator  90  may be comprised of a frame correlator  90 A, an IP correlator  90 B, a TCP correlator  90 C and an application correlator  90 D. These correlators may derive the unique characteristics from the data packets and match the unique characteristics of corresponding data packets.  
         [0048]    In one embodiment, frame correlator  90 A may correlate network specific frame data packets by deriving and comparing the cyclical redundancy checksum (“CRC”) from each network specific frame data packet. Similarly, IP correlator  90 B may correlate internet protocol data packets by deriving and comparing the IP header checksum from each internet protocol data packet. Additionally, TCP correlator  90 C may correlate transport control protocol data packets by deriving and comparing the TCP header checksum from each transport control protocol data packet. Finally, application correlator  90 D may correlate application data packets by comparing strings of data contained within each data packet.  
         [0049]    For example, application correlator  90 D may select the entire data portion of the data packet as a string segment to be used for comparison with a corresponding string segment from a corresponding data packet. In one embodiment, the longer the string segment used for comparison by the application correlator  90 D, the more robust the correlation between two matching data packets.  
         [0050]    [0050]FIG. 5 is a flowchart illustrating a process for intercepting pre-processed data packets on a network according to an embodiment of the invention. A pre-processed data packet may be a data packet that has been handled by any number of network appliances yet has not been processed by the particular UUT whose latency is being calculated.  
         [0051]    For example, a data packet destined for a particular UUT may be detected by a network tap, as illustrated in step  200 . In this fashion, the network tap may intercept the data packet and copy the data packet, as shown in step  202 . Once the data packet has been copied, in step  204 , the network tap may forward the data packet along the network to its ultimate destination. Finally, the network tap may send the copy of the data packet to the LMD for correlation, as illustrated in step  206 .  
         [0052]    [0052]FIG. 6 is a flowchart showing a conventional method for a network appliance to process a data packet received from a network. Accordingly, a network appliance may process a data packet according to the data packet&#39;s protocol layer. For example, a network specific frame data packet may simply be routed to its destination by a router appliance while a data packet comprising a portion of an application data stream may be interpreted and responded to by a Web server appliance.  
         [0053]    In step  210 , a network appliance, or UUT, may receive the data packet. Accordingly, in step  212  the UUT may process the data packet based upon the nature of the UUT as a network appliance (e.g. the function of the UUT) and the protocol level of the data packet. Once the data packet has been processed, the UUT may send the processed data packet along the network to its ultimate destination, as illustrated in step  214 .  
         [0054]    In an alternative embodiment, a data packet may arrive at the UUT from a particular network interface and depart from the UUT from the same network interface. For example, notwithstanding a plurality of simultaneous network connections in place at the UUT, a data packet may arrive on a first network connection and be processed by the UUT and then subsequently depart from the same first network connection.  
         [0055]    [0055]FIG. 7 is a flowchart illustrating a process for intercepting processed data packets on a network according to an embodiment of the invention. A processed data packet may be a data packet that has been handled by the particular UUT whose latency is being calculated. A processed data packet may also have been handled by any number of network appliances in addition to the particular UUT whose latency is being calculated.  
         [0056]    For example, a data packet that has been processed by a particular UUT may be detected by a network tap, as illustrated in step  220 . In this fashion, the network tap may intercept the processed data packet and copy the data packet, as shown in step  222 . Once the processed data packet has been copied, in step  224 , the network tap may forward the processed data packet along the network to its ultimate destination. Finally, the network tap may send the copy of the processed data packet to the LMD for correlation, as illustrated in step  226 .  
         [0057]    [0057]FIG. 8 is a flowchart illustrating a process for calculating a latency for a particular network appliance based on the timestamps of matching data packets according to an embodiment of the invention. Initially, in step  230 , the latency measurement device, or LMD, receives a data packet. In one embodiment, the data packet is received from a network tap through a network interface card.  
         [0058]    Once the data packet has been received, the LMD may timestamp the data packet, as illustrated in step  232 . In one embodiment, the LMD may append the timestamp data to the data packet itself. For example, a wrapper may be placed around the data packet that contains the timestamp information. Alternatively, the LMD may store the timestamp data in a location separate from the data packet. For example, a supplemental database may be maintained by the LMD in memory or on permanent storage to allow the LMD to save the timestamp information generated for each data packet.  
         [0059]    Data packets that have been received and timestamped may be queued for correlation, as shown in step  234 . In one embodiment, each data packet may be queued according to its protocol level. For example, a data packet that comprises a network specific frame may be queued for correlation with other network specific frame data packets.  
         [0060]    In an alternative embodiment, queuing of data packets may include identifying a unique characteristic for each data packet. For example, a network specific frame data packet, an IP data packet, and a TCP data packet each contain a unique checksum value in the packet header information. This unique checksum may be consulted by the LMD to identify the type of data packet that has been received so that the data packet may be appropriately queued for correlation.  
         [0061]    Continuing the example, a string segment from the content of a data packet may be employed as a unique characteristic for the data packet. Although such a string segment is not guaranteed to be unique, a high probability of accurate correlation may still exist. Additionally, to increase the probability of a unique string segment, multiple segments from a plurality of data packets spanning an application data stream may be combined.  
         [0062]    In one embodiment, the unique characteristic of the data packet (e.g. checksum value or string segment) may be coupled with the timestamp information and included in wrapper placed around the data packet by the LMD. Alternatively, this information may be separately stored by the LMD in association with the particular data packet.  
         [0063]    Processed data packets that correspond to the data packets already received and queued by the LMD may also be received by the LMD, as illustrated in step  236 . In step  238 , the corresponding processed data packets may be timestamped and then queued for correlation, as shown in step  240 . Correlation of the data packets with corresponding processed data packets may be accomplished in step  242  by matching the unique characteristic on one data packet with the unique characteristic of its corresponding data packet. Once the corresponding data packets have been matched with each other, a latency for the particular UUT may be calculated based on the timestamps for each data packet, as illustrated in step  244 .  
         [0064]    For example, a pre-processed data packet may be received by the LMD and determined to be an IP packet. A timestamp for the IP packet may be stored in memory available to the LMD. And the data packet may be queued for correlation. The same packet, after being processed by the UUT, may be received by the LMD and similarly timestamped and queued for correlation. The LMD may then match the unique checksum of the pre-processed data packet with the unique checksum of the processed data packet. Once the correlation between the two data packets has been established, a latency for the UUT may be calculated by determining the amount of time the lapsed between the timestamp for the pre-processed data packet and the timestamp for the processed data packet. In one embodiment, a plurality of latency calculations over time may provide an average latency for the particular UUT.  
         [0065]    [0065]FIG. 9 is a flowchart illustrating a process for correlating and matching data packets according to an embodiment of the invention. As described previously with reference to FIG. 8 and shown in step  250 , the process may begin once the LMD has received a data packet. Upon receiving a data packet, the LMD may determine the type of data packet that has been received. In one embodiment, the LMD may determine the type of data packet that has been received by determining the protocol layer of the packet.  
         [0066]    For example, the LMD may determine that the data packet comprises a network specific frame as seen in step  252 . Examples of network specific frames may include ethernet frames, token ring frames, and any other type of network specific frame known to one having ordinary skill in the art. Additionally, the LMD may determine that the data packet comprises an IP datagram, as shown in step  254 , or a transport protocol packet as illustrated in step  256 . Furthermore, the LMD may determine that the data packet comprises a message or a stream of data, as depicted in step  258 . In such a case, the data packet may be considered part of the application protocol layer.  
         [0067]    Once a data packet has been identified, for example by analyzing the protocol layer of the data packet, a unique characteristic of the data packet may also be determined for later use in correlation. In one embodiment, a checksum value included in a data packet may be used as a unique characteristic for that data packet. For example, network specific frames may have a cyclical redundancy checksum value included as part of the frame. The LMD may obtain this value as the unique characteristic of the data packet, as illustrated in step  260 .  
         [0068]    Additionally, an IP datagram or a TCP packet may include a checksum value as part of the header for the data packet. The LMD may obtain the checksum value from the header of an IP datagram, as shown in step  262 , or from the header of a TCP packet as shown in step  264 . This value may then be used as the unique characteristic for the corresponding data packet.  
         [0069]    Although a checksum value may be computed based on the content of the data packet, it may not be absolutely unique. However, the probability of uniquely identifying a data packet based on a checksum value increases as the entropy in the data packet increases. In other words, when a data packet contains more variable content, the unique quality of a checksum value for that data packet similarly increases.  
         [0070]    Additional unique characteristics of a data packet may also be determined for later use in correlation by the LMD. For example, correlation of data packets at any level may employ certain heuristics to capture unique portions of a data packet&#39;s protocol header. In some cases, partial matches of the protocol header or the content of the data packet may be considered. In one embodiment, a network address translation (“NAT”) table may be maintained to uniquely identify data packets based on a source or destination address. Furthermore the existence of certain flags or options bits in a data packet may be used to derive a unique identifier or characteristic of a data packet.  
         [0071]    Alternatively, a contiguous fixed string segment of characters included in the content of a data stream may be used as a unique characteristic. For example, the LMD may obtain a stream segment as the unique characteristic of a data packet, as illustrated in step  266 . In one embodiment, the stream segment may be obtained by performing a Boyer-Moore fixed string stream comparison or a similar fixed string stream comparison.  
         [0072]    Although a fixed string segment of characters from a data stream may not be absolutely unique, the probability of uniquely identifying the data stream based on a fixed string segment increases as the size of the string segment and the variable nature of the string segment increase. Furthermore, additional heuristics such as a NAT lookup table may be used to reduce the set of potential stream match candidates.  
         [0073]    For example, some local networks may implement an internal addressing scheme that may assign non-unique or otherwise invalid global IP addresses to the nodes in the local network. A router linking the local network to the outside network may employ a NAT table to translate the invalid local address to a valid global address. For example, a two element table entry may include (invalid local address, valid global address). Thus, communications directed to the valid global address would be translated to the invalid local address in the NAT table so that the correct node in the local network would receive the packet.  
         [0074]    In certain cases, the NAT table may use a three element table entry to translate traffic between the local network and the outside network. This may advantageously allow the router to overload the valid global addresses and thereby allow more local network nodes to communicate with the outside network. For example, a three element table entry may include (invalid local address, valid global address, outside network address). Thus, communications directed to the valid global address would identified as originating from the outside network address and then be translated to the invalid local address in the NAT table so that the correct node in the local network would receive the packet.  
         [0075]    In step  290 , data packets that have been received by the LMD may be correlated. For example, data packets that have been identified at the same protocol layer may have their unique characteristics compared with each other until a match is found. In one embodiment, two data packets that have matching unique characteristics may be deemed correlated.  
         [0076]    Upon correlating two data packets, a latency for the UUT may be calculated based on the respective timestamps of the correlated data packets, as illustrated in step  292 . For example, the difference between the timestamp for the first data packet and the timestamp for the second data packet may be calculated as the latency for the UUT. Once a latency has been calculated, the process may end.  
         [0077]    Alternatively, the average latency calculation for a particular UUT may include several latency calculations for different pairs of correlated data packets. Similarly, an average latency calculation may be obtained for data packets that belong to certain protocol layers. For example, if the UUT were a Web server, a separate average latency may be calculated for the Web server by incorporating only those latencies calculated for application layer protocol data packets.  
         [0078]    [0078]FIG. 10 is a flow diagram showing an exemplary data packet in transmission between two networks and through a latency measurement device according to an embodiment of the invention. Data packet  110  may originate from network  10  and may travel in the data flow direction toward network  20 . UUT  30  is positioned between network  10  and network  20  such that data packet  110  may traverse UUT  30  during transmission between network  10  and network  20 . Data packet  110 A represents data packet  110  once it has been processed by UUT  30 .  
         [0079]    Tap  80  may advantageously be physically situated such that it detects data packet  110  prior to data packet  110  being processed by UUT  30 . Upon detecting data packet  110 , tap  80  may copy data packet  110  prior to forwarding data packet  110  on the network toward UUT  30  and the ultimate destination for data packet  110 . Additionally, tap  80  may send the copy of data packet  110  to LMD  100 .  
         [0080]    Upon receiving the copy of data packet  110 , LMD  100  may timestamp data packet  110 , as illustrated in flow step  102 . In one embodiment, LMD  100  may apply a timestamp directly to the data packet  110 . For example, a wrapper may be put around the data packet that stores at least timestamp information. Alternatively, LMD  100  may store a timestamp for data packet  110  in memory. The memory used by LMD  100  may be a volatile short term memory or a more permanent memory type such as hard disk drive storage.  
         [0081]    Once LMD  100  has received and timestamped data packet  110 , the data packet is queued for correlation. For example, LMD  100  may internally pass data packet  110  to correlator  90 . In one embodiment, correlator  90  may determine the protocol layer of data packet  110  and queue the packet accordingly. Additionally, LMD  100  may elicit a unique identifier from data packet  110  for later use in the correlation process.  
         [0082]    As described above, UUT  30  receives data packet  110  from tap  80 . Once UUT  30  has processed data packet  110  it sends processed data packet  110 A down the network toward its destination. Advantageously, tap  80 A may be situated such that it detects each processed data packet sent by UUT  30 . For example, tap  80 A may be physically integrated with UUT  30  such that each departing data packet is detected by tap  80 A. Alternatively, tap  80 A may be communicatively coupled with a network cable such that each departing data packet is detected by tap  80 A.  
         [0083]    Upon detecting data packet  110 A, tap  80 A may copy data packet  110 A prior to forwarding data packet  110 A on the network toward its ultimate destination. Additionally, tap  80 A may send the copy of data packet  110 A to LMD  100 . Upon receiving the copy of data packet  110 A, LMD  100  may timestamp data packet  110 A, as illustrated in flow step  102 A. In one embodiment, LMD  100  may apply a timestamp directly to the data packet  110 A, as described above. Alternatively, LMD  100  may store a timestamp for data packet  110 A in memory, as also described above.  
         [0084]    Once LMD  100  has received and timestamped data packet  110 A, the data packet is queued for correlation. For example, LMD  100  may internally pass data packet  110 A to correlator  90 . In one embodiment, correlator  90  may determine the protocol layer of data packet  110 A and queue the packet accordingly. Additionally, LMD  100  may elicit a unique identifier from data packet  110 A for later use in the correlation process.  
         [0085]    Correlator  90 , once it has received data packet  110  and data packet  110 A, may consult the unique identifiers for data packets  110  and  110 A. In one embodiment, when the unique identifiers for two data packets match, then those two data packets are correlated. Once Correlator  90  has correlated two data packets, LMD  100  may calculate a latency for the particular UUT  30  under test. For example, LMD  100  may compare the timestamps of correlated data packets  110  and  110 A to determine the latency of UUT  30 .  
         [0086]    [0086]FIG. 11 is a block diagram illustrating a conventional protocol layering principle widely used in TCP/IP networking environments. Messages passed from a first computer to a second computer may first travel down the protocol layers of the first computer, then travel across a network, and then travel up the protocol layers of the second computer.  
         [0087]    For example, a communication from an application running on a first computer originates in application layer  300 . This communication may be passed by the application as message  302  to the transport layer  304 . The transport layer  304  may pass the message  302  as packet  306  to the internet layer  308 . The internet layer  308  may then pass the packet  306  as datagram  310  to the network interface layer  312 . The network interface layer  312  may then pass the datagram  310  as network specific frame  314  to the physical network  316 .  
         [0088]    The network specific frame  314  may travel across the physical network  316  or across multiple physical networks  316  to its destination in a second computer. Upon reaching its destination, the identical frame  314  may be received at the network interface layer  312 . The network interface layer  312  may then pass the frame  314  as datagram  310  to the internet layer  308 . The internet layer  308  may then pass the datagram  310  as packet  306  to the transport layer  304 . The transport layer  304  may then pass the packet  306  as message  302  to application layer  300  where the message is received as a communication in an application. Frame  314 , datagram  310 , packet  306  and message  302  are identical when traveling between the protocol layers in a TCP/IP networking environment.  
         [0089]    [0089]FIG. 12 is a flow diagram illustrating a conventional technique for demultiplexing incoming data packets, or frames, based on a protocol type found in the frame header. Communication protocols employ multiplexing and demultiplexing techniques between protocol layers in TCP/IP networking environments. For example, when sending a communication, the source computer may include additional information such as the message type, originating application, and protocols used. Eventually, all messages are placed into network frames for transfer and combined into a stream of data packets. At the receiving end, the destination computer uses the additional information in the network frame to guide the processing of the communication.  
         [0090]    For example, in step  320 , a frame arrives at the destination computer. Once the frame has been received, the frame is parsed to determine the frame&#39;s particular type, as illustrated in step  322 . A frame may be one of a variety of frame types. Example frame types include, but are not limited to, address resolution protocol (“ARP”), internet protocol (“IP”), and reverse address resolution protocol (“RARP”).  
         [0091]    Once the frame type has been determined, the content of the frame is passed to a module that is capable of processing the datagram. For example, an ARP datagram may be passed to ARP module  324  for processing. Alternatively, if the frame type indicated an IP datagram, the IP datagram may be passed to IP module  326  for processing up to the next layer in the protocol stack. Additionally, a RARP datagram may be passed to RARP module  328  for processing.  
         [0092]    [0092]FIG. 13 is a flow diagram illustrating a conventional technique for demultiplexing incoming datagrams based on a type found in the IP datagram header. Similar to the processing of frames, IP datagrams may be parsed to determine how to process the particular datagram. For example, in step  330  an IP datagram arrives and is routed to the appropriate module for processing. IP module  326  may parse the datagram to determine the datagram type. Example datagram types include, but are not limited to, internet control message protocol (“ICMP”), user datagram protocol (“UDP”), transport control protocol (“TCP”), and exterior gateway protocol (“EGP”).  
         [0093]    Once the datagram type has been determined, IP module  326  may select a protocol handler for the packet included in the datagram. For example, an EGP datagram may be forwarded to EGP handler  332 . Similarly, an ICMP datagram may be forwarded to ICMP handler  334  while a TCP datagram may be sent to TCP handler  336  for processing up to the next layer in the protocol stack. Additionally, a UDP datagram may be sent to UDP handler  338  for processing.  
         [0094]    [0094]FIG. 14 is a flow diagram illustrating a conventional technique for demultiplexing incoming messages based on a type found in the TCP packet header. Similar to the processing of frames and datagrams, TCP messages may be parsed to determine which application is suited to receive the particular message type. For example, in step  340  a TCP message arrives and is routed to the TCP handler  336  for the appropriate processing. TCP handler  336  may parse the message to determine the message type and the particular originating application.  
         [0095]    Example message types include, but are not limited to, hyper text transfer protocol (“HTTP”), file transfer protocol (“FTP”), and simple mail transfer protocol (“SMTP”). An extensive set of applications are commercially available for use with these and other message types. For example, Netscape Navigator and Microsoft Explorer are applications that use HTTP messages; WS_FTP is an application that uses FTP messages, and Eudora and Microsoft Outlook are applications that use SMTP messages. Additional examples of applications are well known, although not mentioned herein.  
         [0096]    Once the message type has been determined by TCP handler  336 , the message may be routed to the appropriate application for processing. For example, an HTTP message may be forwarded to HTTP application  342 . Similarly, an FTP message may be forwarded to FTP application  344  while an SMTP message may be sent to SMTP application  346  for processing by the application and possibly delivery to an end user.  
         [0097]    [0097]FIG. 15 is a block diagram illustrating an exemplary computer system  350  in which elements and functionality of the present invention are implemented according to one embodiment of the present invention. The present invention may be implemented using hardware, software, or a combination thereof and may be implemented in a computer system or other processing system. Various software embodiments are described in terms of exemplary computer system  350 . After reading this description, it will become apparent to a person having ordinary skill in the relevant art how to implement the invention using other computer systems, processing systems, or computer architectures.  
         [0098]    The computer system  350  includes one or more processors, such as processor  352 . Additional processors may be provided, such as an auxiliary processor to manage input/output, an auxiliary processor to perform floating point mathematical operations, a special-purpose microprocessor having an architecture suitable for fast execution of signal processing algorithms (“digital signal processor”), a slave processor subordinate to the main processing system (“backend processor”), an additional microprocessor or controller for dual or multiple processor systems, or a coprocessor. It is recognized that such auxiliary processors may be discrete processors or may be integrated with the processor  352 .  
         [0099]    The processor  352  is connected to a communication bus  354 . The communication bus  354  may include a data channel for facilitating information transfer between storage and other peripheral components of the computer system  350 . The communication bus  354  further provides the set of signals required for communication with the processor  352 , including a data bus, address bus, and control bus (not shown). The communication bus  354  may comprise any known bus architecture according to promulgated standards, for example, industry standard architecture (ISA), extended industry standard architecture (EISA), Micro Channel Architecture (MCA), peripheral component interconnect (PCI) local bus, standards promulgated by the Institute of Electrical and Electronics Engineers (IEEE) including IEEE 488 general-purpose interface bus (GPIB), IEEE 696/S-100, and the like.  
         [0100]    Computer system  350  includes a main memory  356  and may also include a secondary memory  358 . The main memory  356  provides storage of instructions and data for programs executing on the processor  352 . The main memory  356  is typically semiconductor-based memory such as dynamic random access memory (DRAM) and/or static random access memory (SRAM). Other semiconductor-based memory types include, for example, synchronous dynamic random access memory (SDRAM), Rambus dynamic random access memory (RDRAM), ferroelectric random access memory (FRAM), and the like, as well as read only memory (ROM).  
         [0101]    The secondary memory  358  may include a hard disk drive  360  and/or a removable storage drive  362 , for example a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive  362  may read from and write to a removable storage unit  364  in a well-known manner. Removable storage unit  364  may be, for example, a floppy disk, magnetic tape, optical disk, etc. which may be read from and written to by removable storage drive  362 . Additionally, the removable storage unit  364  may include a computer usable storage medium with computer software and computer data stored thereon.  
         [0102]    In alternative embodiments, secondary memory  358  may include other similar means for allowing computer programs or other instructions to be loaded into the computer system  350 . Such means may include, for example, interface  370  and removable storage unit  372 . Examples of secondary memory  358  may include semiconductor-based memory such as programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable read-only memory (EEPROM), or flash memory (block oriented memory similar to EEPROM). Also included are any other interfaces  370  and removable storage units  372  that allow software and data to be transferred from the removable storage unit  372  to the computer system  350  through interface  370 .  
         [0103]    Computer system  350  may also include a communication interface  374 . Communication interface  374  allows software and data to be transferred between computer system  350  and external devices, networks or information sources. Examples of communication interface  374  include but are not limited to a modem, a network interface (for example an Ethernet card), a communications port, a PCMCIA slot and card, an infrared interface, and the like.  
         [0104]    Communication interface  374  preferably implements industry promulgated architecture standards, such as Ethernet IEEE 802 standards, Fibre Channel, digital subscriber line (DSL), asymmetric digital subscriber line (ASDL), frame relay, asynchronous transfer mode (ATM), integrated digital services network (ISDN), personal communications services (PCS), transmission control protocol/Internet protocol (TCP/IP), serial line Internet protocol/point to point protocol (SLIP/PPP), and so on. Software and data transferred via communication interface  374  may be in the form of signals  378  which may be electronic, electromagnetic, optical or other signals capable of being received by communication interface  374 . These signals  378  are provided to communication interface  374  via channel  376 . Channel  376  carries signals  378  and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, a radio frequency (RF) link, or other communications channels.  
         [0105]    Computer programming instructions (also known as computer programs, software, or firmware) may be stored in the main memory  356  and the secondary memory  358 . Computer programs may also be received via communication interface  374 . Such computer programs, when executed, enable the computer system  350  to perform the features of the present invention. In particular, execution of the computer programming instructions may enable the processor  352  to perform the features and functions of the present invention. Accordingly, such computer programs represent controllers of the computer system  350 .  
         [0106]    In this document, the term “computer program product” is used to refer to any mediun used to provide programming instructions to the computer system  350 . Examples of certain media include removable storage units  364  and  372 , a hard disk installed in hard disk drive  360 , and signals  378 . Thus, a computer program products may be a means for providing programming instructions to the computer system  350 .  
         [0107]    In an embodiment where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system  350  using hard disk drive  360 , removable storage drive  362 , interface  370  or communication interface  374 . The computer programming instructions, when executed by the processor  352 , may cause the processor  352  to perform the features and functions of the invention as described herein.  
         [0108]    In another embodiment, the invention may be implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (“ASICs”). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons having ordinary skill in the relevant art.  
         [0109]    In yet another embodiment, the invention may be implemented using a combination of both hardware and software. It is understood that modification or reconfiguration of the computer system  350  by one having ordinary skill in the relevant art does not depart from the scope or the spirit of the present invention.  
         [0110]    While the particular method and apparatus for passively calculating the latency for a network appliance herein shown and described in detail is fully capable of attaining the above described objects of this invention, it is understood that the description and drawings represent the presently preferred embodiment of the invention and are, as such, a representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art, and that the scope of the present invention is accordingly limited by nothing other than the appended claims.