Abstract:
A packet probe for a packet network accurately generates and monitors packets within the network. The packet probe supports packet generation and packet transmission. When a packet is ready for transmission, a hardware-based time stamp unit affixes a time stamp to the packet reflecting an actual transmission time. The packet probe also supports receiving, filtering, and time stamping received packets. When a packet is received, a packet filter determines whether the received packet should be stored in memory along with a time stamp reflecting an actual reception time.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention relate generally to a method and apparatus for monitoring packet networks and, more specifically, to a more precise way of time stamping packets that are used in measuring transit delay and transit delay variations between two points in a packet network 
         [0003]    2. Description of the Related Art 
         [0004]    Time and frequency alignment are essential to certain types of systems operating in a conventional communications network. For example, time alignment is required by instrumentation systems gathering data at specific time intervals or operating machinery according to specific timing. Frequency alignment is required in time-division multiplexing (TDM) and media streaming systems that require fixed video or audio sample rates across multiple clients. Typically, a system that is properly time-aligned is also frequency aligned. However, frequency alignment typically does not imply time alignment. 
         [0005]    One approach known in the art that provides both time and frequency alignment involves computing an aligned time signal based on global positioning system (GPS) satellite timing signals, which are each held in precise alignment with a global clock reference. Using GPS signals to achieve time or frequency alignment is generally quite expensive and requires a client system to be able to receive satellite time signals from GPS satellites. In general, a more cost effective approach to time alignment is to transmit timing alignment information via a protocol that is operable within a given communications network. 
         [0006]    In conventional TDM networks a physical layer methods implement frequency alignment throughout the network, starting with a designated master clock system. The designated master clock system delivers (frequency) timing information via bit-timing (or symbol-timing) information associated with downstream physical communication links. In normal operation each system coupled to the master clock system replicates the master clock timing information to downstream systems by replicating physical layer timing from the master clock system to each downstream system. Each system within the TDM network receives (frequency) timing information and aligns local (frequency) timing to an upstream clock reference, thereby enabling every system within the TDM network to achieve frequency alignment. 
         [0007]    While frequency alignment within conventional TDM networks is relatively straightforward, packet-switched networks, such as networks based on the popular Ethernet industry standards, present time and frequency alignment challenges because packet-switched networks are not conventionally designed to provide precise delivery time for data or precise timing at any lower protocol levels. A key difference is that the switching and multiplexing functions are not as deterministic as circuit switching and TDM, but have a statistical aspect as well. The statistical nature of switching and multiplexing adds a different notion of quality of service. Whereas error performance is always important, the notions of delay variation and available bandwidth now come into play. For a given packet flow, such as for a circuit-emulated service, a certain minimum “bit rate” may be specified along with a measure of how much more bandwidth can be made available, depending on the level of network congestion. A Service Level Agreement (SLA) between the network provider and an end-user would specify, among other items, the guaranteed (minimum) bit rate (or equivalent) as well as the upper limit to packet delay variation and other factors that could be in jeopardy in situations of network congestion. 
         [0008]    Furthermore, packet-switched networks typically involve multiple nodes that may store and forward data packets, potentially introducing significant transit delay variation between any two points. To generally overcome certain time alignment challenges inherent in packet-switched networks, certain time alignment protocols based on the industry standard internet protocol (IP) have been developed and deployed. One IP-based time alignment protocol is known in the art as the Network Time Protocol (NTP). NTP is used for aligning time between a master time reference and one or more clients. Precision Time Protocol (PTP) is a second IP-based time alignment protocol for aligning one or more client devices to a master time reference. PTP is defined in detail within the IEEE 1588® standard. 
         [0009]    Lightly loaded packet-switched networks typically present relatively low transit delay variation, allowing IP-based alignment protocols such as NTP and PTP to easily achieve excellent accuracy relative to each protocol&#39;s specification. For example, in a lightly loaded gigabit Ethernet-based network, PTP can theoretically provide alignment of better than one hundred nanoseconds. However, conventional networks typically have a wide range of bandwidth loading conditions, which leads to large transit delay variations. Large transit delay variations can potentially cause client devices to fall out of alignment and fail. A network probe may be used to monitor network conditions with respect to a specified protocol, such as NTP or PTP, and to generate alerts when prevailing network conditions do not support proper operation of the specified protocol. With an appropriate alert, network operators can potentially take action to mitigate or avoid a failure. A network probe may also be used to generate traffic simulating a large population of time alignment client devices for the purpose of testing a given packet-switched network&#39;s ability to perform under load prior to operating the network with normal production traffic. 
         [0010]    A conventional network probe comprises a computer system configured to communicate and interact with a time reference server and act as one or more client devices. In PTP, the time reference server is called a grandmaster. When a computer system configured to operate as a network probe interacts with a grandmaster, each incoming packet generates an interrupt within the computer system. An operating system controlling the computer system schedules an interrupt handler to process packets in response to the interrupt. When a PTP packet is received, the interrupt handler is typically configured to act as a PTP client device. The process of scheduling and executing an interrupt may take hundreds of microseconds on a typical computer system with relatively efficient interrupt handling. As a result, the potential measurement error associated with time-stamping an incoming PTP packet may be orders of magnitude larger than the resolution needed for the desired measurement. Time stamps for out-bound packets will include similarly large errors. Furthermore, the computer system may not be able to accurately generate a useful volume of PTP traffic to properly simulate a set of client devices. 
       SUMMARY OF THE INVENTION 
       [0011]    Embodiments of the present invention sets forth a more precise way to time stamp packets that are used in measuring transit delay and transit delay variations between two points in a packet network, and a packet probe that enables such precise time stamping. As a result, transit delay and transit delay variations between two points in a packet network can be measured more precisely. These measurements can then be used in carrying out a number of different network management functions. One such function is congestion monitoring. Certain metrics computed from the delay measurements provided using embodiments of the invention can describe the level of congestion in the network and armed with this knowledge suitable network management actions can be taken such as routing traffic around islands of congested segments. Another use is the development of routing algorithms with “cost of routing” based not on number of hops but on the ability of links to carry real-time traffic or traffic that is sensitive to transit delay variation such as services used for transport of timing. Examples of real-time traffic are VoIP (Voice over Internet Protocol), Video-over-IP, and IPTV (Internet Protocol Television); and examples of timing traffic are PTP and NTP. 
         [0012]    A method of transmitting a packet onto a physical layer of a packet network, according to an embodiment of the present invention, includes the steps of generating a packet template having a plurality of data fields including a time stamp data field, updating the data fields of the packet template including the time stamp data field in hardware, the time stamp data field being updated last, and transmitting the packet template with all of the data fields updated as a completed packet. The time stamp data field is updated based on a start time of the step of transmitting and a time offset, wherein the time offset is an estimate of a delay between the start time of the step of transmitting and an actual time the completed packet is transmitted onto a physical layer of the packet network. 
         [0013]    A method of processing a packet received from a physical layer of a packet network, according to an embodiment of the present invention, includes the steps of receiving packets through a media access control unit, and storing at least one received packet in memory with an associated time stamp, wherein the associated time stamp is generated in hardware and has a time value equal to a reference time at the time the received packet is stored in memory minus an estimate of a delay through the media access control unit. Prior to time stamping, the received packets may be filtered based on packet type. For some types of received packets, such as timing packets, a response packet may be automatically generated and transmitted to a sender of the received packet. A timing packet is generally defined as a packet that follows the structure rules associated with an IP protocol associated with packet-based timing methods such as PTP and NTP. Any packet that has a field for inserting a time stamp can be used as a timing packet. 
         [0014]    A packet probe according to an embodiment of the present invention includes a transmit module coupled to a reference time source and configured to time stamp a transmit packet in hardware with a first time value, and a receive module coupled to the reference time source and configured to time stamp a receive packet in hardware with a second time value. The first time value is offset from a time indicated by the reference time source at the time the transmit packet is time stamped. The amount of this offset is representative of a delay through a transmit media access control unit by which the transmit packet is transmitted onto a physical layer of the packet network. The second time value is offset from a time indicated by the reference time source at the time the receive packet is time stamped. The amount of this offset is representative of a delay through a receive media access control unit by which the receive packet is received from the physical layer of the packet network. 
         [0015]    Other embodiments include, without limitation, a computer-readable medium that includes instructions that enable a processing unit to implement one or more aspects of the disclosed methods as well as a system configured to implement one or more aspects of the disclosed methods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0017]      FIG. 1  illustrates a network system configured to implement one or more aspects of the present invention. 
           [0018]      FIG. 2  illustrates a time protocol interaction between a master clock and a slave clock communicating via a packet network, according to one embodiment of the invention. 
           [0019]      FIG. 3  illustrates a time protocol IP packet used to communicate between the master clock and the slave clock within the packet network, according to one embodiment of the invention. 
           [0020]      FIG. 4  illustrates a packet probe engine within the network probe, according to one embodiment of the invention. 
           [0021]      FIG. 5A  illustrates an Ethernet physical layer transmitter unit configured to generate a physical layer signal that represents Ethernet frame data. 
           [0022]      FIG. 5B  illustrates an Ethernet physical layer receiver unit configured to receive and decode a physical layer signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention. 
         [0024]    Monitoring of packets and transit delay and transit delay variations between two points in a packet-switched network is achieved in the embodiments of the present invention described herein by deploying suitable gear, known as a network probe, at designated points in the network. The network probe may be provided as a stand-alone equipment or its monitoring functionality may be embedded into network elements. 
         [0025]    Flow monitoring using network probes provides information regarding packet flows between selected points in the network. As a result, problem links and network elements can be identified, and corrective action such as suitable routing table modifications can be taken. The absolute delay and packet delay variation between pairs of network probes can be used to establish the health of network segments. Moreover, multiple streams between a pair of network probes, where each stream is assigned a different priority or class, known in the art as Quality of Service (QoS) class or Class of Service (Cos), are monitored to provide guidance as to the behavior of the packet network segment for each of the different classes. 
         [0026]      FIG. 1  illustrates a network system  100  configured to implement one or more aspects of the present invention. The network system  100  comprises a grandmaster  110  coupled to a global positioning system (GPS) time receiver  112  and to a packet-switched network  120 , a network probe  130  coupled to a GPS time receiver  132  and to the packet-switched network  120 , and network clients  140  coupled to the packet-switched network  120 . 
         [0027]    The packet-switched network  120  is configured to forward IP packets between two or more attached devices, based on a destination field within the IP packets. The packet network comprises network elements  122 , which are configured to forward internet protocol (IP) packets based on an IP address, an Ethernet destination address, any other technically feasible forwarding information, or any combination thereof. 
         [0028]    The GPS time receiver  112  computes a GPS time signal  114  based on time signals received from a plurality of GPS satellites. The grandmaster  110  receives the GPS time signal  114  and uses the GPS time signal  114  as a master clock when responding to time alignment protocol requests, such as PTP requests. The grandmaster  110  may be connected to the packet-switched network  120  via a physical port  150 , which is provided by network element  122 - 1 . The physical port  150  may comprise an Ethernet port, or any other technically feasible type of network port. 
         [0029]    The GPS time receiver  132  computes GPS time signal  134  based on time signals received from a plurality of GPS satellites. During normal operation the GPS time signals  114  and  134  are precisely aligned in time (synchronized). The network probe  130  receives GPS time signal  134  for use as a local time reference when performing network performance measurements, as discussed in greater detail below. 
         [0030]    The network probe  130  is connected to the packet-switched network  120  via physical port  152 , which is provided by network element  122 - 4 . By measuring network performance available through network element  122 - 4 , the network probe  130  is generally able to determine network performance for other devices also attached to network element  122 - 4 . For example, network clients  140 - 1  and  140 - 2  should experience network performance characteristics according to measurements taken by the network probe  130 . Physical ports  152 ,  154 - 1 , and  154 - 2  may comprise Ethernet ports, or any other technically feasible type of network port. In certain embodiments, physical ports  152 ,  154 - 1 , and  154 - 2  should be substantially identical in design. For example physical ports  152 , 154 - 1 , and  154 - 2  may comprise gigabit Ethernet ports. 
         [0031]    In this configuration, IP packets transmitted by the network probe  130  and destined for the grandmaster  110  traverse network elements  122 - 4 ,  122 - 2 , and  122 - 1 . Similarly, IP packets transmitted by the grandmaster  110  and destined for the network probe  130  traverse network elements  122 - 1 ,  122 - 2 , and  122 - 4 . Congestion within any network element  122 - 1 ,  122 - 2 ,  122 - 4  can impact delivery time of packets traversing between the grandmaster  110  and network probe  130 . Such congestion may result from traffic arriving from unrelated sources, such as transit traffic from network element  122 - 3 , which passes through network element  122 - 2  before final delivery. 
         [0032]    With GPS time  134  available to network probe  130  and GPS time  114  available to grandmaster  110 , network probe  130  can perform accurate transit delay measurements by acting as a PTP client of grandmaster  110 . An accurate transit delay measurement can be used to characterize end-to-end network performance for the packet-switched network  120 , and, more specifically, to characterize performance of the packet-switched network  120  with respect to end-to-end performance between network element  122 - 1  and network element  122 - 4 . Accurately measuring end-to-end transit delay through the packet-switched network  120  is possible because the master clock  210  includes an accurate departure time stamp with outgoing packets, and the slave clock  212  is able to perform accurate arrival time measurements. 
         [0033]      FIG. 2  illustrates a time protocol interaction between a master clock  210  and a slave clock  212  communicating via packet-switched network  120 , according to one embodiment of the invention. The master clock  210  corresponds to grandmaster  110 , while the slave clock  212  represents the network probe  130 , network client  140 , or any other technically feasible network client. 
         [0034]    Event A  220  represents a departure of a first time stamped packet at actual departure time T 1  from the master clock  210 . A binary representation of the actual departure time T 1  is included as the time stamp for the first time stamped packet. Event B  222  represents an arrival of the first time stamped packet at actual arrival time T 2  at the slave clock  212 . Actual arrival time T 2  is equal to measured arrival time τ 2  plus a measurement error ε 2 . With GPS time signal  134  providing a highly accurate local time reference with which to measure arrival time, measurement error ε 2  can be very small (essentially zero) in practice and the measured arrival time τ 2  can accurately represent the actual arrival time T 2 . 
         [0035]    Event C  224  represents a departure of a second time stamped packet at actual departure time T 3  from the slave clock  212 . A binary representation of actual departure time T 3  is included as the time stamp for the second time stamped packet. Actual departure time T 3  is equal to measured departure time τ 3  plus a measurement error ε 3 . As with measuring T 2 , the GPS time signal  134  provides a highly accurate local time reference, driving measurement error ε 3  to essentially zero. Event D  226  represents an arrival of the second time stamped packet at actual arrival time T 4  by the master clock  210 . 
         [0036]    The first time stamped packet traverses the packet-switched network  120  in master-slave transit delay time TMS. The second time stamped packet traverses the packet-switched network  120  in slave-master transit delay time TSM. Transit delay times TMS and TSM are representative of transit delay within packet-switched network  120  between network elements  122 - 1  and  122 - 4 . Persons skilled in the art will recognize that transit delay times TMS and TSM may also be used to characterize other aspects of packet-switched network  120 , such as loading or congestion. 
         [0037]      FIG. 3  illustrates a time protocol IP packet  300  used to communicate between the master clock  210  and the slave clock  212  within the packet-switched network  120 , according to one embodiment of the invention. As shown, the time protocol IP packet  300  comprises standard IP fields, such a protocol field  312 , a source IP address field  320 , a destination IP address field  322 , and a time stamp field  330 . The protocol field  312  identifies how a recipient device should interpret the packet  300  with respect to a specific protocol. For example, the protocol field  312  may identify the packet  300  as a PTP packet, thereby uniquely defining other fields within the packet  300 , such as a time stamp field  330 , used to indicate a departure time for the packet. A source IP address field  320  identifies an IP address for the system that sent the packet  300 , while a destination IP address  322  identifies an IP address to which the packet  300  should be delivered by packet-switched network  120 . 
         [0038]      FIG. 4  illustrates a packet probe engine (PPE)  400  within the network probe  130 , according to one embodiment of the invention. The PPE  400  comprises a transmit module  430 , a receive module  460 , a scheduler module  410 , and a command and status registers module  412 . The transmit module  430 , receive module  460 , scheduler module  410 , and command and status registers module  412  are coupled to a control bus  405 , configured to allow a host processor (not shown) to communicate with each respective module. 
         [0039]    The PPE  400  generates a transmit data signal (Tx)  428  and a receive data signal (Rx)  458  based on a reference time signal  424 , a physical transmission start signal (Phy Tx)  422 , a transmit latency compensation signal (Tx Compensation)  426 , a physical reception start signal (Phy Rx)  452 , and a receive latency compensation signal (Rx Compensation)  456 . The reference time signal  424  is a globally accurate and aligned time signal derived from the GPS time signal  134  of  FIG. 1 . 
         [0040]    The transmit latency compensation signal  426  is a constant transmit latency value characterizing a delay through a transmitter media access control unit (shown in  FIG. 5A ). Tx  428  passes through this transmitter media access control unit before it is transmitted onto the physical layer. Phy Tx  422  is asserted at a transmission time offset relative to an actual start of transmission by the physical layer transmitter unit. The transmission time offset is characterized by the transmit latency compensation signal  426 . The transmit latency compensation signal  426  may also include processing latency associated with the transmit module  430 . 
         [0041]    The receive latency compensation signal  456  is a constant value characterizing a delay of Rx  458  through a receiver media access control unit (shown in  FIG. 5B ). Rx  458  is received at the receive module  460  from the physical layer by way of this receiver media access control unit. Phy Rx  452  is asserted at a receive time offset relative to an actual start of reception by the physical layer receiver unit. The receive time offset is characterized by the receive latency compensation signal  456 . The receive latency compensation signal  456  may also include processing latency associated with the receive module  460 . 
         [0042]    The transmit latency compensation signal  426  and the receive latency compensation signal  456  may be determined from prior measurements of the delay through the transmitter media access control unit and the receiver media access control unit, respectively. In some embodiments, the delay from a prior packet may be sampled and applied as the time offset for a current packet. 
         [0043]    The transmit module  430  is configured to generate an IP packet and transmit the IP packet as an Ethernet frame via Tx  428  in response to a generate command sent via send control  420  requesting that the transmit module  430  generate and transmit the IP packet. The transmit module  430  comprises a packet build unit  432 , a client random access memory (RAM)  435 , a packet template RAM  436 , a protocol RAM  437 , a time stamp unit  433 , and a transmitter unit  434 . In one embodiment, the packet build unit  432  is implemented as a field programmable gate array (FPGA) and the time stamp unit  433  is implemented as an FPGA. In alternative embodiments, the packet build unit  432  and the time stamp unit  433  may be implemented as other types of hardware devices including application specific integrated circuits. 
         [0044]    The packet build unit  432  performs a set of operations to build a complete protocol packet and prepare the packet for transmission via the transmitter  434 . Packets are built based on a template system, where a major portion of a given packet payload is defined in a static template. When a given packet is built, updates are applied to data fields within the template to generate a complete and correct packet. In one embodiment, there are two types of packet updates: stream-based updates and protocol-based updates. For stream-based updates, an identifier for each updated field and data associated with each updated field are specified by software executing on the host processor on a stream-by-stream basis. For protocol-based updates, an identifier for each updated field and data associated with each updated field are specified for each particular type of message. For protocol-based updates, each update is applied to all packets of a given type. Updates may also include client or session related information, such as client IP address, VLAN address, and packet sequence number. During the packet build process, the PPE is responsible for calculating and updating any required checksums. In one embodiment, the packet build unit  432  performs checksum computations for packets built by the PPE  400 . 
         [0045]    Data field updates can be applied on a per packet-stream (session) basis when a packet is being built. The packet-stream update mechanism for generating packets should update one or more fields in a packet that vary on a per stream basis. One example of a data field that needs to be updated on a per packet-stream basis is the destination MAC address in an Ethernet header. Similarly, the destination IP address in an IP header of a packet needs to be updated to reflect a client (destination) IP address for a packet being generated. The packet-stream update mechanism can be used to update any field in the packet. For efficient memory usage, certain update information may be shared over multiple streams, whereas data pertaining to a specific stream should include one instance of the stream-specific data. 
         [0046]    The client RAM  435  is configured to store client related information, such as client IP address, VLAN address, and sequence number. The template RAM  436  is configured to store a definition of a basic packet structure to be generated and includes data fields that are populated with specific client and protocol information during packet generation. The protocol RAM  437  is configured to store information related to a protocol structure, such as where specific data fields are placed within a generated packet. In one embodiment, client RAM  435 , template RAM  436 , and protocol RAM  437  are implemented as FPGA memory block RAMs. 
         [0047]    In the embodiment of the present invention described herein, the template RAM  436  and protocol RAM  437  are organized as two pages, and one of the two pages may be designated as active, making the other page inactive. The active page is used by the PPE  400  to perform packet generation processes. The inactive page may be accessed by the host processor to configure a new template. The inactive page may be designated the active page by the host processor at any time, however, the new designation will only take place at a safe time, such as when the packet build unit  432  is idle. 
         [0048]    The time stamp unit  433  receives reference time  424  and the transmit latency compensation signal  426  and generates a compensated transmit time stamp, which is transmitted to the packet build unit  432 . The compensated transmit time stamp is generated by adding the reference time  424  to the transmit latency compensation signal  426 . The compensated transmit time stamp is the last update performed during packet generation. 
         [0049]    The transmitter unit  434  receives packet information from the packet build unit  432  and transmits the packet information as Tx  428 , which is supplied to an Ethernet media access control (MAC) unit, shown in  FIG. 5A . The transmitter unit  434  is configured to perform any data and transfer rate translation necessary to generate Tx  428 . 
         [0050]    The receive module  460  is configured to receive an IP packet encoded as an Ethernet frame via Rx  458  and filter the IP packet according to a set of acceptance rules. If an IP packet passes the acceptance rules, then certain data from the IP packet is made available to the control bus  405 . 
         [0051]    The receive module  460  comprises a packet filter  462 , a time stamp unit  464 , a receive data first-in first-out (FIFO) buffer  466 , check units  468 , a hash table unit  470 , and a client identification (ID) unit  472 . The receive module  460  is configured to receive an incoming packet via Rx  458  and to process the packet according to programmable settings. In an alternative embodiment, the receive data FIFO  466  may be configured as a dual-port RAM. Also, the time stamp unit  464  is implemented in one embodiment as an FPGA. In alternative embodiments, the time stamp unit  464  may be implemented as other types of hardware devices including application specific integrated circuits. 
         [0052]    The packet filter  462  is configured to identify incoming packets that match one of a plurality of programmable patterns. For packets that match one of the plurality of programmable patterns, the receive module  460  responds according to a programmable set of rules. One programmable response is to forward the packet to software executing on the host processor. Another response is to drop (discard) the packet. Yet another response is to generate an automatic reply to a respective client via the transmit module  430 . The packet filter  462  is coupled to a set of check units  468  that are each configured to recognize a particular packet type using a set of programmable rule sets. The check units  468  and any associated rule sets may each be configured by software executing on the host processor. Any technically feasible technique may be used by the check units  468  to recognize packets. 
         [0053]    When a packet is recognized as a packet that should be handled by the PPE  400 , certain data fields within the packet are extracted and pushed into the receive data FIFO  466 , along with a corresponding compensated receive time stamp. The receive data FIFO  466  stores the extracted packet data and presents the extracted packet data to a hash table unit  470 , which is configured to identify long patterns and generate shorter index values using any technically feasible techniques. Persons skilled in the art will recognize that hashing techniques may be used to generate a relatively short index value from a longer data string. For example, an IPv4 address may comprise a data string of thirty-two bits of address information that may be hashed into a twelve-bit index value that concisely identifies one of up to 2048 different client devices (via their IP address) communicating with the PPE  400 . Similarly, an IPv6 address may comprise a data string of 128 address bits that may be hashed to an arbitrary length index value. In one embodiment, the hash table unit  470  hashes an IPv4 IP address and related session information into a twelve bit value to identify up to 2048 different sessions from up to 2048 different IP addresses. 
         [0054]    The time stamp unit  464  receives reference time  424  and the receive latency compensation signal  456  and generates the compensated receive time stamp, which is transmitted to the receive data FIFO  466 . The compensated receive time stamp is generated by adding the reference time  424  to the receive latency compensation signal  456 . The compensated receive time stamp is pushed into the receive data FIFO  466  along with related packet information for packets that are identified by the packet filter  462  as needing to be processed by the PPE  400 . 
         [0055]    The client identification unit (ID)  472  receives an index value corresponding to a packet that was identified by the packet filter  462  for processing by the PPE. The client ID  472  uses the index value to retrieve certain client and session state information used to generate a response. For example, the index value may be used to retrieve a packet sequence number, a protocol type, and so forth, which are necessary to properly respond. In one embodiment, the receive module  460  requests that the scheduler module  410  generate a response when appropriate within the context of a particular protocol. The request may be generated via the host processor or directly from the receive module  460  interacting with the scheduler module  410 . 
         [0056]    The scheduler module  410  is configured to trigger packet generation by the transmit module  430  according to a programmable set of rules. The scheduler module  410  triggers the transmit module  430  to generate and transmit packets at programmed rates to specific streams in timing applications and to deliver probe packets at designated intervals for probe applications. The scheduler module  410  uses the notion of events for purposes of scheduling, an event being the delivery of a packet for a particular stream. The number of scheduled events that can be supported is a function of hardware complexity. Supporting up to 2048 scheduled events is quite straightforward with current technological constraints. Each scheduled event is defined by data stored in an event entry within a control memory, disposed within the PPE  400 . In one embodiment, the control memory resides within the scheduler module  410 . In an alternative embodiment, the control memory resides within the command and status registers module  412 . 
         [0057]    The intervals of transmission are programmable. In keeping with most applications that may require this feature, a typical design will support intervals of transmission based on powers of two with a range of values between 2 −10  to 2 7  seconds, or 1/1024 to 128 seconds. 
         [0058]    Each event has a programmable interval. Also, each event is programmed with pointers to information stored within the PPE  400  regarding the packet content and the type of packet to send. The packet build unit  432  builds and transmits the packet when it is scheduled. Packet generation and transmission may also be scheduled according to a dithering process. When enabled, dithering will vary the transmit intervals of all active streams (packets associated with a given IP session), except a certain stream programmed in a specified entry (e.g., event entry 2048 of 2048 possible entries) of the control memory. A transmit window of approximately 800 microseconds may be used for scheduling dithered transmission of packets within a 976 microsecond (1024 Hz) scheduling interval. Although the time between successive transmissions of a specific stream will be varied, the average rate will be executed within each scheduling interval exactly as programmed. Extensions of the dithering process allow the launching of packets in a burst mode and in modes that have multiple packet transmissions, unequally spaced in time, but following an overall periodic behavior. This transmission pattern is often called “N packets in M seconds” and is useful to excite certain modes of behavior in packet networks. 
         [0059]    The command and status registers module  412  are configured to store certain configuration information for controlling the operation of the PPE  400  and modules therein. For example, the configuration information for the schedule module  410  may be stored within the command and status registers module  412 . Additionally, a selection bit can be stored within the command and status registers module  412  for controlling which pages of memory within the template RAM  436  and protocol RAM  437  are active. 
         [0060]      FIG. 5A  illustrates an Ethernet physical layer transmitter unit (Tx PHY)  512  configured to generate a physical layer signal  520  that represents Ethernet frame data Tx  428 . A transmitter media access control unit (Tx MAC)  510  processes Tx  428  and sends processed Tx  428  data to Tx PHY  512  for transmission via physical layer signal  520 . The Tx MAC  510  manages transmission of the Ethernet frames via the Tx PHY  512 . The physical layer signal  520  represents Ethernet frames as sequential bit patterns in either an electrical or optical media. 
         [0061]    A preamble pattern marks the start of each transmitted Ethernet frame. At the start of the preamble pattern, Phy Tx  422  is asserted, alerting the time stamp unit  433  of  FIG. 4  that an outbound Ethernet frame is departing. The latency from when a frame actually starts to when Phy Tx  422  is asserted may be large compared to a measured time resolution; however latency should be consistent within the Tx PHY  512 . In one embodiment, Phy Tx  422  is sampled by the time stamp unit  464  of  FIG. 4  with a time resolution of 4 nanoseconds. In an alternative embodiment, Phys Tx  422  is sampled by the time stamp unit  464  with a time resolution of 8 nanoseconds. 
         [0062]      FIG. 5B  illustrates an Ethernet physical layer receiver unit (Rx PHY)  532  configured to receive and decode a physical layer signal  540 , according to one embodiment of the invention. The Rx PHY  532  transmits the decoded physical layer signal to a receiver media access control unit (Rx MAC)  530  for processing. The physical layer signal  540  represents Ethernet frames as sequential bit patterns in either an electrical or optical media. 
         [0063]    The beginning of each Ethernet frame is marked by a preamble pattern. When the preamble pattern arrives at the Rx PHY  532 , Phy Rx  452  is asserted, alerting the time stamp unit  464  that an Ethernet frame is arriving. In one embodiment, Phy Rx  458  is sampled by the time stamp unit  464  of  FIG. 4  with a time resolution of 4 nanoseconds. In an alternative embodiment, Phys Rx  458  is sampled by the time stamp unit  464  with a time resolution of 8 nanoseconds. 
         [0064]    In the embodiments of the present invention described above, grandmaster  110  may be any PTP time reference server. In one embodiment, the grandmaster  110  is a network probe that includes PPE  400  and is configured to operate as a PTP time reference server. 
         [0065]    While embodiments of the present invention are described in terms of Ethernet technologies, persons skilled in the art will recognize that this invention may be implemented using any technically feasible physical link layer technology without departing the scope of this invention. 
         [0066]    In sum, a technique for accurately probing a network to measure end-to-end transit delay is disclosed. A network probe incorporates hardware-based time stamping of transmitted packets using a physical transmission start signal (Phy Tx  422 ) and a physical data layer latency compensation figure to accurately time stamp a transmitted IP packet based on an actual start time for transmitting the packet. Each transmitted IP packet may be scheduled for transmission using a hardware scheduler that is configured trigger the packet build control  432  to construct and transmit a packet. The network probe also incorporates hardware-based time stamping of received packets using a physical reception start signal (Phy Rx  452 ) and a physical data layer latency compensation figure to accurately time stamp a received IP packet based on an actual arrival time for the packet. 
         [0067]    One advantage of the disclosed system is that greater accuracy can be achieved through compensated hardware-based time stamping for both received and transmitted IP packets. An additional advantage of the disclosed system is that transmitted packets may be generated to achieve high packet rates, according to precise timing and generation characteristics. 
         [0068]    While the forgoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. For example, aspects of the present invention may be implemented in hardware or software or in a combination of hardware and software. One embodiment of the invention may be implemented as a program product for use with a computer system. The program(s) of the program product define functions of the embodiments (including the methods described herein) and can be contained on a variety of computer-readable storage media. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, flash memory, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. Such computer-readable storage media, when carrying computer-readable instructions that direct the functions of the present invention, are embodiments of the present invention.