Patent Publication Number: US-7898969-B2

Title: Performance measurement in a packet transmission network

Description:
This application claims priority from PCT/FR2005/01111 filed May 3, 2005, and FR Application No. 0404986, filed May 7, 2004, which are hereby incorporated by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of telecommunication networks and more precisely to the field of the metrology of packet transmission telecommunication networks. 
     Metrology, in the literal sense of “science of measurement”, is expanding in different network fields such as traffic characterization and modeling, traffic analysis and quality of service and performance optimization. Network metrology is also used to improve network supervision. One particular objective is to provide assistance with the dimensioning of a network and with diagnosing problems detected in a network. 
     The increasing complexity of networks, and in particular of the Internet, leads to a lack of knowledge of traffic and conditions of use. It is becoming more and more difficult to have real control of the network and its behavior. 
     The traffic of a network is generally made up of a plurality of streams of packets. A stream of packets is defined as an exchange of data between two terminals of a network the packets whereof have common characteristics, in particular common source, destination and service characteristics. 
     Metrology generally has the objective of determining measurements to be performed on the traffic of the network under study in order to obtain a better knowledge of that traffic. 
     Two main types of measurements are known. A first type is made up of measurements performed on a stream of test packets. To perform these so-called “active” measurements, a stream of test packets is sent across the network being studied from a sender first terminal to a receiver second terminal. A test packet generally has a specific format containing a first field indicating a time reference of sending the packet and a second field indicating the sequence number of the packet in the stream of test packets sent. Consequently, the receiver terminal determines a transmission lead time of a test packet from the sender terminal as a function of the first field of the packet. The receiver terminal can also detect losses of packets as a function of the second field of a test packet. More generally, a test packet is a packet whereof a sending time reference is known and that can be distinguished from other packets on reception in order to associate it with an arrival time reference or to determine that it has been lost. 
     It is thus possible to obtain performance characteristics of an “end-to-end” stream, i.e. of the complete path of the stream of test packets, from the sender to the receiver, and in particular measurements of the transmission lead time between the two terminals, on the basis of the known sending and receiving time references of the packets. 
     This first type of measurement therefore produces performance measurements on a complete path of the stream of test packets, i.e. between the sender terminal of the stream and the receiver terminal of the stream. The measurements obtained in this way relate to the test packets of a stream. Because of this they are qualitatively accurate. 
     One drawback of this type of measurement is that it provides information relating only to the extremities of the stream of test packets. Thus it is impossible to obtain information on a segment of the complete path between two network equipments or between a network equipment and one of the terminals. Information obtained in this way is therefore geographically inaccurate. 
     There is known a second type of measurement performed in a network equipment by an analysis of the streams passing in transit through it. These so-called “passive” measurements can be performed either by onboard measurement units in the network equipments or by measurement units (or passive probes) external to the network equipments and dedicated to passive measurements. The latter generally have less information available than onboard measurement units. These measurement units monitor the traffic circulating on the links between the network equipments. This type of passive measurement, widely used in existing networks, produces volumetric information for each stream and for each network equipment. 
     One drawback of this type of measurement is that it supplies information relating to a network equipment that is very difficult to correlate with other information relating to another network equipment. Consequently, even though it is possible to obtain relatively accurate topological information on a given segment by correlation, such correlation remains complex to implement, in particular if the streams are sampled in order to analyze them. 
     Another drawback of this type of measurement is that it provides only volumetric information relating to the stream for each network equipment, in particular the number of analyzed packets and the sum of the sizes of the analyzed packets for a given stream. Consequently, the information obtained by this type of measurement may prove to be qualitatively insufficient and even somewhat unreliable, in particular if the streams are sampled in order to analyze them. 
     It is thus beneficial to obtain qualitatively accurate information, i.e. information relating to the packets of a stream, and geographically accurate information, i.e. information relating to segments of the complete path of the stream in the existing networks. 
     Note that the expression “network equipment” refers to an active network equipment, i.e. one that fulfills an active processing function in the network, such as switches or routers, for example. An active processing function in the network may also be defined as the opposite of a passive processing function fulfilled in particular by a measurement unit. 
     SUMMARY OF THE INVENTION 
     The present invention aims to propose a solution tending to satisfy these requirements. 
     A first aspect of the invention proposes a measurement method for use in a data packet transmission network, wherein a stream of packets of data sent by a first terminal passes in transit through at least one network equipment with which there is associated a stream measurement unit; wherein the first terminal and said measurement unit are connected to a collection unit; said method comprising the following steps:
         the first terminal generates a packet stream according to a predetermined rule;   the measurement unit analyzes at least one packet of said stream passing through the network equipment;   the first terminal dispatches to the collection unit a description of a stream of sent packets, comprising a list including at least sizes of the sent packets;   the stream measurement unit dispatches to the collection unit a stream description, comprising at least a sum of the sizes of the analyzed packets; and   the collection unit identifies each analyzed packet as a function of the stream description and of said predetermined rule so as to correlate, packet by packet, said stream description and said packet stream description.       

     A second aspect of the invention proposes a measurement system comprising means adapted to implement the above method. 
     A third aspect of the invention proposes a collection unit comprising means adapted to implement the above method. 
     Thanks to the above arrangements, it is possible to obtain geographically and qualitatively accurate information on traffic in the transmission network under study. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, objects and advantages of the invention will become apparent on reading the description of one of its embodiments. 
       The invention will also be better understood with the aid of the drawings, in which: 
         FIG. 1  is a diagram of a network architecture as claimed in one embodiment of the invention; 
         FIG. 2  shows a measurement method as claimed in one embodiment of the invention; 
         FIG. 3  shows a measurement method as claimed in one embodiment of the invention in a network architecture based on a collection unit comprising first and second modules. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention may be used in all fields of metrology applied to packet transmission networks. 
     The invention proposes to couple active measurements and passive measurements in order to obtain the benefits of each of them. Accordingly, in one embodiment of the invention, two terminals (or active probes) exchange a test packet that is analyzed at several points of the network, in particular on the basis of time references for the passage of the packet. 
     The context of the invention is that of any type of network for transmission of packets of fixed size and packets of variable size. To illustrate the following description, an IP (Internet Protocol) network is taken by way of example, without limiting the scope of the invention. The invention may in particular be implemented under version 4 or version 6 of the Internet Protocol. 
     The invention applies to analysis methods comprising selecting packets from the streams analyzed, for example sampling the streams, or to analysis methods with no selection of packets from the streams analyzed. 
     Note that, in one embodiment of the invention, a measurement unit generates “stream descriptions” comprising information relating to the stream whereas a terminal generates “packet stream descriptions” comprising information relating to the packets of the stream. 
       FIG. 1  represents a network architecture comprising a measurement system as claimed in one embodiment of the invention. A terminal  11  and a terminal  12  are connected to the IP network  10 . The terminals  11  and  12  are adapted to generate streams of test packets. The IP network  10  comprises equipments  13 ,  14 ,  15 ,  16  and  17 . Of these equipments, the equipments  13 ,  15  and  16  are each provided with an onboard stream measurement unit. The measurement units have a measurement export function. The invention also covers a configuration in which the stream measurement unit is an external entity monitoring traffic in transit on a transmission link between network equipments or between a terminal and an equipment. The terminals  11  and  12  and the equipments comprising stream measurement units  13 ,  15  and  16  are connected to a collection unit  18 . This collection unit is adapted to receive information exported by the measurement units and the terminals  11  and  12  in order to group the information and correlate it to provide accurate information relating to the test packets of the stream and corresponding to particular segments of the complete path of the stream, in particular performance information relating to loss of packets or to the packet transmission lead time. 
     The terminals  11  and  12  may be fixed terminals. They may equally well be mobile terminals. 
     Note that, in an embodiment of the invention shown in  FIG. 3 , the terminals are connected to a first module of a collection unit while the measurement units associated with a network equipment are connected to a second module of a collection unit. In certain situations described hereinafter, this kind of architecture is more advantageous. 
     In a preferred embodiment of the invention, the onboard stream measurement units in the equipments  13 ,  15 ,  16  implement stream analysis functions of the kind, currently being standardized, known as ‘IPFIX’, one implementation of which is distributed by Cisco Systems, Inc. under the name “Netflow”. An “IPFIX” type service is a global analysis service of streams passing in transit through packet transmission network equipments, in particular routers and switches of IP, ATM, Ethernet or MPLS (Multiple Protocol Label Switch) type. This kind of service supplies an analysis of the streams entering an equipment. It is also adapted to accumulate statistical measurements relating to a given stream and to export those measurements asynchronously to a collection unit  18 . The measurements are preferably exported to the collection unit by encoding the data in order to reduce the necessary bandwidth. The measurements may be exported on the fly, on expiry of a timer or on detection of a stream end. A measurement unit may also store these measurements to enable the collection unit  18  to download them. 
     The invention covers any other analysis function of a measurement unit supplying stream information and any other method of exporting that stream information. 
     In the remainder of this description, for reasons of clarity and by way of example, the network equipments provided with a stream measurement unit are routers. The invention also covers a stream measurement system in which the network equipments provided with a stream measurement unit are other network equipments, such as switches, for example. 
     A stream of test packets sent by the terminal  11  to the terminal  12  is generally identified by the following information:
         the IP address, denoted T11_SrcAddr, and the logical port, denoted T11_SrcPort, of the sender terminal  11 ;   the IP address, denoted T12_DstAddr, and the logical port, denoted T12_DstPort, of the receiver terminal  12 ;   the protocol used, denoted Proto_test, for example TCP (standing for “Transmission Control Protocol”), UDP (standing for “User Datagram Protocol”), or ICMP (standing for “Internet Control Message Protocol”).       

     A network equipment provided with a stream measurement unit is in a position to supply stream descriptions that preferably comprise:
         a field for the input and output interface number, respectively denoted ‘Input’ and ‘Output’;   a field for the number of analyzed and accumulated packets for a stream, denoted ‘Packets’;   a field for the number of octets of the IP layer corresponding to the analyzed packets of the stream, denoted ‘Octets’;   a field for a time reference for the passage of the first analyzed packet in the stream and a field for a time reference of the passage of the last analyzed packet in that stream, respectively denoted ‘First’ and ‘Last’;   a field for the IP address of the input interface of the next router, denoted ‘NextHop’.       

     If the equipment is a router, a stream description preferably further comprises a field for the numbers of the administrative domains, or AS (standing for “Autonomous System”), that have been crossed, respectively denoted ‘SrcAS’ and ‘DstAS’. 
     A measurement unit of a router stores data relating to the streams that cross that router. On reception of a packet, the measurement unit determines if the packet belongs to a stream being analyzed. If it belongs to a stream being analyzed, the fields corresponding to the number of analyzed packets for that stream, to the sum of the sizes of the analyzed packets, and to the time reference of the passage of the last analyzed packet, respectively the fields ‘Packets’, ‘Octets’ and ‘Last’, are updated. If the packet received does not belong to a stream being analyzed, the measurement unit of the router initiates an analysis of that stream, storing, among other things, in the ‘First’ field, a time reference for the passage of the first packet of the stream. The measurement unit of the router terminates the analysis of a stream on detection of the end of the stream. 
     A measurement unit is generally adapted to export the stored data either when the end of the stream is detected or before the end of the stream being analyzed if a certain time has elapsed since the initiation of the analysis of the stream. 
     The stored data relating to the analysis of a stream is then encoded in order to export it to the collection unit  18  in the form of stream description tickets. A plurality of stream description tickets relating to a plurality of streams being studied may be grouped into a stream description block, as described hereinafter. 
     In addition to the technical data that the collection unit  18  needs to decode a description ticket, in particular a field denoted ‘Version’ identifying a software version of the IPFIX type service, the header of a stream description ticket also preferably indicates a time reference of the router. Fields denoted ‘SysUptime’ and ‘UnixSecs’ enable deduction of the time at which the stream description ticket was created. A field denoted ‘FlowSequence’ enables detection of loss of stream description tickets. Table 1 below shows the format of the exported stream description tickets in one embodiment of the invention. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Octets 
                 Field name 
                 Description 
               
               
                   
               
             
            
               
                 0-1 
                 Version 
                 IPFIX version number 
               
               
                 2-3 
                 Count 
                 Number of streams exported in this 
               
               
                   
                   
                 packet 
               
               
                 4-7 
                 SysUptime 
                 Number of milliseconds elapsed 
               
               
                   
                   
                 since starting the machine 
               
               
                  8-11 
                 UnixSecs 
                 Number of seconds elapsed since 
               
               
                   
                   
                 Jan. 1, 1970: Unix timestamp 
               
               
                 12-15 
                 UnixNSecs 
                 Number of residual nanoseconds 
               
               
                   
                   
                 since Jan. 1, 1970 
               
               
                 16-19 
                 FlowSequence 
                 Count of total number of streams 
               
               
                   
                   
                 exported 
               
               
                 20-23 
                 Reserved 
                 Reserved octets 
               
               
                   
               
            
           
         
       
     
     In this table the field ‘Reserved’ may be used to describe information such as the sender (engine_‘type’ and id′) and a sampling method (sampling_interval). 
     Each stream is preferably described in a stream description ticket as detailed in table 2 below, comprising in particular information relating to the administrative domains, or AS (standing for “Autonomous System”) and relating to the Type of Service, or ToS, equivalent to a Quality of Service, or QoS. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Field 
                 Octets 
                 Description 
               
               
                   
               
             
            
               
                 SrcAddr 
                 0-3 
                 Source IP address 
               
               
                 DstAddr 
                 4-7 
                 Destination IP address 
               
               
                 NextHop 
                  8-11 
                 IP address of next router 
               
               
                 Input 
                 12-13 
                 SNMP index of the input interface 
               
               
                 Output 
                 14-15 
                 SNMP index of the output interface 
               
               
                 Packets 
                 16-19 
                 Number of analyzed packets in the stream 
               
               
                 Octets 
                 20-23 
                 Total number of octets in the analyzed 
               
               
                   
                   
                 packets of the stream 
               
               
                 First 
                 24-27 
                 Time reference of the stream start 
               
               
                   
                   
                 Stream 
               
               
                 Last 
                 28-31 
                 Time reference of end of stream 
               
               
                 SrcPort 
                 32-33 
                 TCP/UDP source port or equivalent 
               
               
                 DstPort 
                 34-35 
                 TCP/UDP destination port or equivalent 
               
               
                 Pad1 
                 36 
                 Not used (zero) 
               
               
                 TcpFlags 
                 37 
                 TCP flags or cumulative flags 
               
               
                 Proto 
                 38 
                 IP type (example TCP = 6; UDP = 17) 
               
               
                 Tos 
                 39 
                 IP service type 
               
               
                 SrcAS 
                 40-41 
                 Source AS number 
               
               
                 DstAS 
                 42-43 
                 Destination AS number 
               
               
                 SrcMask 
                 44 
                 Source IP address mask 
               
               
                 DstMask 
                 45 
                 Destination IP address mask 
               
               
                 Pad2 
                 46-47 
                 Not used (zero) 
               
               
                   
               
            
           
         
       
     
     Each stream may also be described by a stream description ticket comprising only some of the information listed above. 
     The stream description blocks grouping stream description tickets are generally sent by the measurement units to a collection unit in the form of a UDP datagram. A UDP datagram of 1464 octets, corresponding to an Ethernet frame of 1500 octets, can contain up to 30 stream description tickets. 
     An IPFIX type service, implemented in software, generally cannot analyze all the packets passing in transit through a network equipment. The measurement unit therefore reduces the number of packets to be analyzed by selecting certain packets in the stream of studied packets. The invention covers any way of selecting packets in a stream. In one embodiment of the invention, packets to be analyzed are selected by sampling the studied stream. The measurement units then export the stream information, as detailed in table 2, to the collection unit  18 . 
     As stated above, test packets are injected into the network. In fact, the terminal  11  sends a stream of test packets to the terminal  12  across the network  10 . The test packet stream therefore passes in transit through at least some of the network equipments  10 . When test packets enter network equipments provided with a measurement unit, some or all of the test packets of the stream are analyzed. 
     In one embodiment of the invention, for each stream studied, the stream information from each measurement unit comprises at least the sum of the sizes of the test packets analyzed in the stream. 
     Moreover, the terminals  11  and  12  each send the collection unit  18  a sent packet stream description, respectively a received packet stream description, for the sent, respectively received packets of the stream. This description preferably comprises a list of the sizes of sent test packets, respectively received. This description advantageously further comprises a time reference associated with sending, respectively receiving, each packet, as described in more detail hereinafter. 
     In one embodiment of the invention, the stream of test packets sent by the terminal  11  to the terminal  12  is generated according to a predetermined rule. Indeed, the stream of test packets is composed of a sequence of ordered test packets that are specific in the sense that each test analyzed packet may be identified in the stream as a function of the sum of the size of the test analyzed packets. 
     For example, a stream of n test packets Pi, for i=0 to i=n−1, n being a non-zero integer, is generated so that the size of each packet Pi is a power of 2. Accordingly, the first sent packet P 0  has a size of 2 0 , the second sent packet P 1  has a size of 2 1 , and so on up to the n th  sent packet Pn−1, which has a size of 2 n-1 . The sum of the sizes of the sent packets is therefore 2 n −1. The corresponding binary representation is ‘ . . . 111111’b, with the least significant bit on the right, in which each packet is identified by one bit. Accordingly, a test packet is very simple to identify from the binary representation of the sum of the sizes of the packets of the sequence by its position in the stream sent. 
     It will be noted that to identify the packets analyzed by a given measurement unit, the order of sending the packets is not important. On the other hand, for calculating the packet transmission delay the value in the field ‘First’ is associated with the first analyzed packet and the value in the field ‘Last’ is associated with the last analyzed packet. It is important to know the sending and transmission order to make this association. 
     It will be noted that the invention also covers transmission of test packets in any order, provided that the collection unit is informed of the order of transmission of the test packets. In fact, in one embodiment of the invention, the packet Pi may be transmitted after the packet Pi+1 if the collection unit is informed of this. 
     Accordingly, the collection unit  18  receives on the one hand information relating to the packets sent and received by the terminals, and in particular the list comprising the size of the test packets, via the packet stream descriptions, and on the other hand information relating to the stream, and in particular the sum of the sizes of the analyzed packets, via the stream descriptions. It is consequently in a position to identify each packet that has been analyzed in a measurement unit and thus to calculate, if necessary, a loss of test packets relating to a defined segment between a terminal and an equipment or between two network equipments, as is detailed hereinafter. It will be noted that these latter calculations are possible in the situation where the complete stream of test packets is analyzed or in the situation where only certain packets of the test stream are selected to be analyzed. 
     Moreover, the collection unit  18  further receives time references of sending and receiving test packets via the packet stream descriptions sent by the terminals and a time reference for the passage of the first and last analyzed packets from each of the measurement entities of the network equipments, via the stream descriptions. It is consequently in a position also to provide a packet transmission delay relating to a segment, as described in detail hereinafter. 
     It will be noted that to identify the analyzed packets by a measurement unit, the order of the packets of the stream sent by the terminal  11  is not important. 
     In one embodiment of the invention, the sum of the sizes of the analyzed packets and the time references of the passage of the first and last analyzed packets are sent into the stream descriptions, more precisely into the fields respectively denoted ‘Octets’, ‘First’ and ‘Last’. 
     More generally, the collection unit  18  is in a position to supply the following information instantaneously:
         information relating to the path of the packets of the stream in the network; for example a list of one or more equipments through which the analyzed packets pass in transit;   information relating to management of the equipments of the path of the stream in the network, for example a list of management addresses of one or more equipments through which the analyzed packets pass in transit;   information relating to processing of the stream in the equipments of the path, for example a list of levels of quality of service applied by one or more equipments to the analyzed packets or a list of administrative domains crossed by the stream;   network performance information relating to a loss of packets on a segment between one of the terminals and an equipment or between two terminals and/or a packet transmission lead time relating to a segment between one of the terminals and an equipment or between two equipments and/or to a transmission lead time variation.       

     Under certain conditions it may prove very useful to reiterate successively the steps of sending of test packet streams by the terminal  11 , of stream analysis in the network equipments, and of stream reception by the terminal  12 . Under such conditions, the measurement unit is able to supply, by iteration and by correlation of information obtained on each iteration relating to a segment between one of the terminals and an equipment or between two equipments, measurements of performance statistics such as those defined by the IETF (Internet Engineering Task Force) or the ITU (International Telecommunications Union), for example:
         a loss of packets;   a packet transmission lead time; and   a transmission lead time variation.       

     Reiteration of the measurement process is also very useful if the analysis is performed on selected packets of the test stream, for example by sampling the test stream. 
     Accordingly, after reiteration of the steps, the collection unit is in a position to supply the following information more accurately and more exhaustively, by correlation of the data obtained on each iteration:
         information relating to the path of the packets of the stream in the network, for example a list of one or more equipments through which the analyzed packets pass in transit;   information relating to management of the equipments of the path of the stream in the network, for example a list of management addresses of one or more equipments through which the analyzed packets pass in transit;   information relating to processing of the stream in the equipments of the path, for example a list of levels of equality of service applied by one or more equipments to the analyzed packets or a list of administrative domains crossed by the stream;   network performance information relating to a loss of packets on a segment between one of the terminals and an equipment or between two equipments and/or to a packet transmission lead time relating to a segment between one of the terminals and an equipment or between two equipments.       

     It will be noted that the reiteration of the steps proves to be particularly advantageous if the analysis is effected on packets selected from the test stream, for example by sampling the test stream. In fact, in this case certain instantaneous vectors may provide incomplete information. On the other hand, if the analysis is effected on all the packets of the test stream, the data listed above can be obtained by correlation without reiteration. 
       FIG. 2  shows a measurement method as claimed in one embodiment of the invention. 
     A stream of test packets sent by the terminal  11  to the terminal  12  passes through the equipments  13  and  15 , each of which is provided with a stream measurement unit having an IPFIX type stream information export function. 
     In the following description, the measurement units sample the stream of test packets before analyzing the stream. The invention covers other methods of selecting packets in a stream. 
     In one embodiment of the invention, the measurement units sample, manage and store stream descriptions in a table of stream characteristics. The structure of the entries in this table is analogous to the structure described above with reference to table 2. As claimed in the type of service of the analyzed stream, the functional level of the analysis or the network equipment type, this table may have additional fields or, conversely, some of the fields already list may not be filled in. 
     A network generally comprises an integer number N of equipments. The stream descriptions are sent to the collection unit  18  from one of the IP addresses of the equipment Ei, denoted Ei_SrcAddrMgt. 
     The packets of the IP stream sent by the terminal  11  to the terminal  12  are therefore observed when they enter the equipments Ei. 
     In an embodiment of the invention, the measurement method comprises the following steps:
         step  21 : sending of the sequence of test packets from the terminal  11  to the terminal  12 ;   step  22 ,  22 ′: selection of the test packets and analysis of the stream on the basis of the selected packets by the measurement units of the equipments Ei; during the selection step, each selected test packet is also date-stamped by associating a time reference with it;   step  23 ,  23 ′: analysis of the selected packets and storage in a stream characteristics table entry corresponding to the stream, then storage of a time reference for the passage of the first and last analyzed packets in the fields denoted ‘First’ and ‘Last’;   step  24 : reception of the sequence of test packets by the terminal  12 ;   step  25 : sending of the packet stream description by the terminal  11  to the collection unit  18 ;   step  26 ,  26 ′: on end of stream detection (inactivity of the stream, for example), exporting of the stream descriptions by the measurement units of the equipments Ei to the collection unit  18 ;   step  27 : sending of the packet stream description by the terminal  12  to the collection unit  18 ;   step  28 : grouping and correlation of data relating to this stream by the collection unit  18 ;   step  29 : production of the vectors of the information relating to equipments of the path of the stream or to segments of the path taken by the packets of that stream or else relating to the complete path.       

     Each of the steps of the measurement method cited above is described in detail hereinafter. 
     In the step  21 , the terminal  11  sends the terminal  12  a stream made up of a sequence of packets the sum of the sizes whereof enables the analyzed packets to be identified on the basis of the total number of bytes analyzed. In a preferred embodiment of the invention, the sequence of test packets is a list, denoted T11_Sn, of packets {P 0 , P 1 , P 2 , . . . , Pn−1} and characterized in that the size of each packet Pi is a power of 2, as described previously. Such a list preferably comprises a list of the sizes of packets. 
     The terminal  11  stores the sending time, denoted T11_Pit 0 , of each packet Pi of the stream in order to be in a position to provide a packet stream description. 
     In the step  22 ,  22 ′, a measurement unit of an equipment Ei selects packets from this stream to analyze them. 
     The list of the packets Pk analyzed by an equipment Ei is denoted Ei_Sk. The number of elements in the list Ei_Sk is denoted Ei_Size_Sk and corresponds to the value of the field ‘Packets’ contained in the stream descriptions sent by the equipment Ei to the collection unit  18 . 
     The sum of the sizes of the packets in the list Ei_Sk is denoted Ei_Sum_Sk and corresponds to the value of the field ‘Octets’ contained in the stream descriptions sent by the equipment Ei. 
     It is to be noted that two equipments rarely analyze the same packet. There are other selection techniques, currently being standardized by the IETF (Internet Engineering Task Force), that enable selection of the same packets at different points of passage of the network. 
     In the step  23 ,  23 ′, the stream measurement unit of each equipment Ei analyzes a selected test packet and adds the characteristics of that packet to the characteristics already stored in the stream characteristics table relating to the stream of that packet. 
     The measurement unit manages such a table in which each entry corresponds to a stream. If there is no entry for a given stream, identified by the data T11_SrcAddr, T11_SrcPort, T12_DstAddr, T12_DstPort, Proto_test, then it is created by the following steps:
         a time reference of the passage of the first test analyzed packet in this stream is stored in the field ‘First’ of the table entry;   the IP address T11_SrcAddr of the terminal  11  is stored in the field ‘SrcAddr’ of the table entry;   the TCP or UDP port T11_SrcPort of the terminal  11  is stored in the field ‘SrcPort’ of the table entry;   the IP address of the terminal  12 , T12_DstAddr, is stored in the field ‘DstAddr’ of the table entry;   the TCP or UDP port T12_DstPort of the terminal  12  is stored in the field DstPort of the table entry;   the values of the other fields of the table entry are filled in using routing information or packet switching information.       

     On reception of a packet, the number of octets of the test packet is added to the value of the field ‘Octets’ of the table entry, after which the ‘Packets’ field of the table entry is increased by 1. 
     A time reference of passage of the packet is stored in the field ‘Last’ of the table entry. 
     In the step  24 , the terminal  12  receives the packet stream, certain packets whereof may have been lost. It stores a reception time reference and the size of each packet in order to prepare a received packet stream description. 
     The complete list of the packets Pj received by the terminal  12  is referenced T12_Sj. A list of this kind preferably comprises a list of the sizes of the packets of the test stream. The number of elements in the list T12_Sj is denoted T12_Size_Sj and corresponds to the value of the field ‘Packets’ of the packet stream description produced by the terminal  12 . 
     The sum of the sizes of the packets from the list T12_Sj is denoted T12_Sum_Sj and corresponds to the value of the field ‘Octets’ of the packet stream description produced by the terminal  12 . 
     In the step  25 , the terminal  11  produces a sent packet stream description that it sends to the collection unit  18 . A description of this kind preferably contains at least the following information: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 SrcAddr 
                 T11_SrcAddr 
               
               
                   
                 DstAddr 
                 T12_DstAddr 
               
               
                   
                 NextHop 
                 E1_Addr 
               
               
                   
                 Packets 
                 n 
               
               
                   
                 Octets 
                 2 n −1 
               
               
                   
                 Packets_list 
                 T11_Sn 
               
               
                   
                 times 
                 {T11_P0t0, T11_Plt0, T11_P2t0, . . . , 
               
               
                   
                   
                 T11_Pn−1t0} 
               
               
                   
                 SrcPort 
                 T11_SrcPort 
               
               
                   
                 DstPort 
                 T12_DstPort 
               
               
                   
                 Proto 
                 Proto_test 
               
               
                   
                 Tos 
                 T11_Tos 
               
               
                   
                 SrcAS 
                 T11_SrcAS 
               
               
                   
                 DstAS 
                 T11_DstAS 
               
               
                   
                   
               
            
           
         
       
     
     In the step  26 ,  26 ′, the stream measurement unit of Ei detects the end of the stream from criteria analogous to those of the IPFIX type software, i.e. after a packet absence lead time, or if the memory used to store stream descriptions is lacking. 
     The measurement unit then groups the stream descriptions into stream description blocks and sends them to the collection unit from the management address, denoted Ei_SrcAddrMgt. The IP destination address of such tickets is Ei_DstAddrMgt. 
     In the step  27 , the terminal  12  sends the collection unit a receive packet stream description. A description of this kind preferably comprises the following characteristics: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 SrcAddr 
                 T11_SrcAddr 
               
               
                   
                 DstAddr 
                 T12_DstAddr 
               
               
                   
                 NextHop 
                 0 
               
               
                   
                 Packets 
                 T12_Size_Sj 
               
               
                   
                 Octets 
                 T12_Sum_Sj 
               
               
                   
                 Packets list 
                 T12_Sj 
               
               
                   
                 times 
                 {T12_P1t0, . . . , T12_Pn−1t0} 
               
               
                   
                 SrcPort 
                 T11_SrcPort 
               
               
                   
                 DstPort 
                 T12_DstPort 
               
               
                   
                 Proto 
                 Proto_test 
               
               
                   
                 Tos 
                 T12_Tos 
               
               
                   
                 SrcAS 
                 T12_SrcAS 
               
               
                   
                 DstAS 
                 T12_DstAS 
               
               
                   
                   
               
            
           
         
       
     
     In the step  28 , the collection unit  18  groups the packet stream description tickets and the description tickets of the stream. 
     For the descriptions of a test steam received from an equipment Ei, the collection unit determines the first analyzed packet from the value contained in the field ‘Octets’ corresponding to the sum of the sizes of the analyzed packets, and associates therewith as a passage time reference the value contained in the field ‘First’. The collection unit then determines the last analyzed packet and associates with it as a passage time reference the value contained in the field ‘Last’. It is possible that a single packet may have been analyzed as described in the example detailed hereinbelow. 
     An equipment analyzes three packets a first packet P 2  of size 4 bytes, a second packet P 3  of size 8 bytes and a third packet P 7  of size 128 bytes of the stream of test packets sent by the terminal  11  to the terminal  12 . In this case, the ‘Octets’ field equals 140 i.e. ‘10001100’b in binary representation with the least significant bit on the right. By reading the binary expression, the first analyzed packet, P 2 , and the last one analyzed, P 7 , are identified. 
     If the number Ei_Size_Sk is equal to 1, that means that only one packet has been analyzed and therefore that only one passage time reference is available for this stream in the equipment Ei. 
     In one embodiment of the invention, the collection unit groups all of the received stream data to form the following table: 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Field 
                 T11 
                 . . . 
                 Ei 
                 Ei+1 
                 . . . 
                 T12 
               
               
                   
               
             
            
               
                 SrcAddr 
                 T11_SrcAddr 
                 . . . 
                 T11_SrcAddr 
                 T11_SrcAddr 
                 . . . 
                 T11_SrcAddr 
               
               
                 DstAddr 
                 T12_DstAddr 
                 . . . 
                 T12_DstAddr 
                 T12_DstAddr 
                 . . . 
                 T12_DstAddr 
               
               
                 SrcPort 
                 T11_SrcPort 
                 . . . 
                 T11_SrcPort 
                 T11_SrcPort 
                 . . . 
                 T11_SrcPort 
               
               
                 DstPort 
                 T12_DstPort 
                 . . . 
                 T12_DstPort 
                 T12_DstPort 
                 . . . 
                 T12_DstPort 
               
               
                 Proto 
                 Test_proto 
                 . . . 
                 Test_proto 
                 Test_proto 
                 . . . 
                 Test_proto 
               
               
                 Packets 
                 n 
                 . . . 
                 Ei_Size_Sk 
                 Ei+1_Size_Sk 
                 . . . 
                 T12_Size_Sj 
               
               
                 Octets 
                 2 n −1 
                 . . . 
                 Ei_Sum_Sk 
                 Ei+1_Sum_Sk 
                 . . . 
                 T12_Sum_Sj 
               
               
                 NextHop 
                 E1_Addr 
                 . . . 
                 Ei+1_Addr 
                 Ei+2_Addr 
                 . . . 
                 0 
               
               
                 Tos 
                 T11_Tos 
                 . . . 
                 Ei_Tos 
                 Ei+1_Tos 
                 . . . 
                 T12_Tos 
               
               
                 SrcAS 
                 T11_AS 
                 . . . 
                 Ei_AS or 
                 Ei_AS or 
                 . . . 
                 0 
               
               
                   
                   
                   
                 T11_AS 
                 T11_AS 
               
               
                 DstAS 
                 0 
                 . . . 
                 Ei+1_AS or 
                 Ei+2_AS or 
                 . . . 
                 T12_AS 
               
               
                   
                   
                   
                 T12_AS 
                 T12_AS 
               
               
                 Input 
                 0 
                 . . . 
                 Ei_in 
                 Ei+1_in 
                 . . . 
                 T12_in 
               
               
                 Output 
                 T11_out 
                 . . . 
                 Ei_out 
                 Ei+1_out 
                 . . . 
                 0 
               
               
                 SrcAddrMgt 
                 T11_AddrMgt 
                 . . . 
                 Ei_AddrMgt 
                 Ei+1_AddrMgt 
                 . . . 
                 T12_AddrMgt 
               
               
                 P0_time 
                 T11_P0t0 
                 . . . 
                 Ei_P0t0 
                 Ei+1_P0t0 
                 . . . 
                 T12_P0t0 
               
               
                   
                   
                   
                 or 0 
                 or 0 
                   
                 or lost 
               
               
                 P1_time 
                 T11_P1t0 
                 . . . 
                 Ei_P1t0 
                 Ei+1_P1t0 
                 . . . 
                 T12_P1t0 
               
               
                   
                   
                   
                 or 0 
                 or 0 
                   
                 or lost 
               
               
                 P2_time 
                 T11_P2t0 
                 . . . 
                 Ei_P2t0 
                 Ei+1_P2t0 
                 . . . 
                 T12_P2t0 
               
               
                   
                   
                   
                 or 0 
                 or 0 
                   
                 or lost 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 Pn−1_time 
                 T11_Pn−1t0 
                 . . . 
                 Ei_Pn−1t0 
                 Ei+1_Pn1t0 
                 . . . 
                 T12_Pn−1t0 
               
               
                   
                   
                   
                 or 0 
                 or 0 
                   
                 or lost 
               
               
                   
               
            
           
         
       
     
     In the step  29 , in one embodiment of the invention, depending on the configuration of the collection unit, the latter may supply:
         information relating to the path of the packets of the stream in the network;   information relating to the management of the equipments of the path of the stream in the network;   information relating to processing of the stream in the equipments of the path;   network performance information relating to a loss of packets on a segment between one of the terminals and an equipment or between two equipments and/or to a packet transmission lead time relating to a segment between one of the terminals and an equipment or between two equipments.       

     It can also calculate the packet transmission lead times and the packet losses on the complete path of the stream, as shown in the examples described hereinafter. 
     From the step  28 , in one embodiment of the invention, the collection unit groups the Next Hop information of the analyzed packets as shown in the table below: 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 NextHop 
                 T11 
                 . . . 
                 Ei 
                 Ei+1 
                 . . . 
                 T12 
               
               
                   
               
             
            
               
                 P0 
                 E1_Addr 
                 . . . 
                 Ei_Addr 
                 — 
                 . . . 
                 0 
               
               
                 P1 
                 E1_Addr 
                 . . . 
                 — 
                 — 
                 . . . 
                 0 
               
               
                 P2 
                 E1_Addr 
                 . . . 
                 Ei_Addr 
                 — 
                 . . . 
                 — 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 Pn−1 
                 E1_Addr 
                 . . . 
                 Ei_Addr 
                 Ei+1_Addr 
                 . . . 
                 0 
               
               
                   
               
            
           
         
       
     
     The collection unit supplies a NextHop instantaneous vector of the stream: 
     {E1_Addr, Ei_Addr, Ei+1_Addr, 0}. 
     Over a short time period, the production of Next Hop instantaneous vectors at the time of the successive measurements quickly determines the Next Hop of each equipment Ei and therefore the complete vector: 
     {T11_AddrSrc; E1_Addr, E2_Addr, . . . , Ei_Addr, Ei+1_Addr, . . . , T12_AddrDest}. 
     Over a long period, and after the determination of the Next Hop complete vector, comparing the instantaneous vectors with the complete vector is a very quick way to detect modifications of the path taken. 
     From the step  28 , in one embodiment of the invention, the collection unit  18  groups the source addresses used by the measurement units to export the stream descriptions to the collection unit, as shown in the following table. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 SrcAddrMgt 
                 T11 
                 Ei 
                 Ei+1 
                 T12 
               
               
                   
               
             
            
               
                 P0 
                 T11_AddrMgt 
                 Ei_AddrMgt 
                 — 
                 T12_AddrMgt 
               
               
                 P1 
                 T11_AddrMgt 
                 — 
                 — 
                 T12_AddrMgt 
               
               
                 P2 
                 T11_AddrMgt 
                 Ei_AddrMgt 
                 — 
                 — 
               
               
                 Pn−1 
                 T11_AddrMgt 
                 Ei_AddrMgt 
                 Ei+1_AddrMgt 
                 T12_AddrMgt 
               
               
                   
               
            
           
         
       
     
     The collection unit supplies a management address instantaneous vector: 
     {T11_AddrMgt, Ei_AddrMgt, Ei+1 AddrMgt, T12_AddrMgt}. 
     Over a short period, the production of management address instantaneous vectors at the time of successive measurements quickly determines the management address of each Ei and therefore a complete vector: 
     {T11_AddrMgt, E1_AddrMgt . . . Ei_AddrMgt, Ei+1_AddrMgt, . . . , T12_AddrMgt}. 
     From the step  28 , in one embodiment of the invention, the collection unit groups the AS information of the analyzed packets, as shown in the tables below, for the source AS and the destination AS. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 SrcAS 
                 T11 
                 . . . 
                 Ei 
                 Ei+1 
                 . . . 
                 T12 
               
               
                   
               
             
            
               
                 P0 
                 T11_AS 
                 . . . 
                 Ei−1_AS 
                 — 
                 . . . 
                 — 
               
               
                 P1 
                 T11_AS 
                 . . . 
                 — 
                 — 
                 . . . 
                 — 
               
               
                 P2 
                 T11_AS 
                 . . . 
                 Ei−1_AS 
                 — 
                 . . . 
                 — 
               
               
                 Pn−1 
                 T11_AS 
                 . . . 
                 Ei−1_AS 
                 Ei_AS 
                 . . . 
                 — 
               
               
                   
                   
                 . . . 
                   
                   
                 . . . 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 DstAS 
                 T11 
                 . . . 
                 Ei 
                 Ei+1 
                 . . . 
                 T12 
               
               
                   
               
             
            
               
                 P0 
                 — 
                 . . . 
                 Ei+1_AS 
                 — 
                 . . . 
                 T12_AS 
               
               
                 P1 
                 — 
                 . . . 
                 — 
                 — 
                 . . . 
                 T12_AS 
               
               
                 P2 
                 — 
                 . . . 
                 Ei+1_AS 
                 — 
                 . . . 
                 — 
               
               
                 Pn−1 
                 — 
                 . . . 
                 Ei+1_AS 
                 T12_AS 
                 . . . 
                 T12_AS 
               
               
                   
                   
                 . . . 
                   
                   
                 . . . 
               
               
                   
               
            
           
         
       
     
     The collection unit supplies an administrative domain instantaneous vector: 
     {T11_AS, Ei_AS, Ei_AS, Ei+1_AS, T12_AS}. 
     Over a short period, the production of administrative domain instantaneous vectors at the time of the successive measurements quickly determines the source and destination administrative domains of the stream in each equipment Ei. Thus a complete vector is obtained: 
     {T11_AS, E1_AS, . . . , Ei_AS, Ei+1_AS, . . . , T12_AS}. 
     Over a long period, and after the measurement of the complete vector, comparing the instantaneous vectors with the complete vector is a way to detect immediately modifications of the path taken. 
     The ToS or QoS field (or DSCP in IPv4 or its equivalent in IPv6) determines the level of priority of a packet in a queue of a router (or other equipment). This information is very important, in particular in the interdomain context, for the verification of the provision of the devices for marking the quality of service. 
     From the step  28 , in one embodiment, the collection unit groups the QoS information of the analyzed packets as shown in the table below. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 QoS 
                 T11 
                 . . . 
                 Ei 
                 Ei+1 
                 . . . 
                 T12 
               
               
                   
               
             
            
               
                 P0 
                 T11_QoS 
                 . . . 
                 Ei_QoS 
                 — 
                 . . . 
                 0 
               
               
                 P1 
                 T11_QoS 
                 . . . 
                 — 
                 — 
                 . . . 
                 0 
               
               
                 P2 
                 T11_QoS 
                 . . . 
                 Ei_QoS 
                 — 
                 . . . 
                 — 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 Pn−1 
                 T11_QoS 
                 . . . 
                 Ei_QoS 
                 Ei+1_QoS 
                 . . . 
                 0 
               
               
                   
               
            
           
         
       
     
     The collection unit  18  supplies a QoS instantaneous vector: 
     {T11_QoS, Ei_QoS, Ei+1_QoS, T12_QoS}. 
     Over a short period, the production of QoS field instantaneous vectors at the time of the successive measurements quickly determines the QoS field of each Ei and therefore a complete vector: 
     {T11_QoS, E1_QoS, . . . , Ei_QoS, Ei+1_QoS, . . . , T12_QoS}. 
     Over a long period, and after measuring the complete vector, comparing the instantaneous vectors with the complete vector is a way to detect immediately modifications of the QoS field of the stream. This comparison in particular verifies that the QoS field has not changed, even if the path has changed. 
     From the step  28 , in one embodiment, the collection unit groups the instantaneous passage time references of each analyzed packet: 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 11 
               
               
                   
               
               
                 Passage 
                   
                   
                   
                   
                   
                   
               
               
                 time 
               
               
                 reference 
                 T11 
                 . . . 
                 Ei 
                 Ei+1 
                 . . . 
                 T12 
               
               
                   
               
             
            
               
                 P0_time 
                 T11_P0t0 
                 . . . 
                 Ei_P0t0 
                 — 
                 . . . 
                 T12_P0t0 
               
               
                 P1_time 
                 T11_P1t0 
                 . . . 
                 — 
                 — 
                 . . . 
                 — 
               
               
                 P2_time 
                 T11_P2t0 
                 . . . 
                 Ei_P2t0 
                 — 
                 . . . 
                 — 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 Pn−1_time 
                 T11_Pn−1t0 
                 . . . 
                 Ei_Pn−1t0 —   
                 Ei+1_Pn−1t0 
                 . . . 
                 T12_Pn−1t0 
               
               
                   
               
            
           
         
       
     
     The collection unit then calculates instantaneous times between the different points of the network at which the same packet Pi has been analyzed. The table below details the times per segment, relative to each packet: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 12 
               
               
                   
               
               
                 Time 
                 T11 −&gt;Ei 
                 Ei −&gt; Ei+1 
                 Ei+1 −&gt; T12 
               
               
                   
               
             
            
               
                 P0 
                 Ei_P0t0 − T11_P0t0 
                 — 
                 — 
               
               
                 P1 
                 — 
                 — 
                 — 
               
               
                 P2 
                 Ei_P2t0 − T11_P2t0 
                 — 
                 — 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 Pn−1 
                 Ei_Pn−1t0 − 
                 Ei+1_Pn−1t0 − 
                 T12_Pn−1t0 − 
               
               
                   
                 T11_Pn−1t0 
                 Ei_Pn−1t0 
                 Ei+1_Pn−1t0 
               
               
                   
               
            
           
         
       
     
     A packet is not analyzed systematically by all the equipments Ei. 
     Analyzing the packet Pn−1 enables the collection unit to supply an instantaneous transmission lead time vector of the complete path between the terminals  11  and  12 : 
     {Ei+1_Pn−1t0-T11_Pnt0, Ei+1_Pn−1t0−Ei_Pn−1t0, T12_Pn−1t0-Ei+1_Pn−1t0}. 
     Analyzing the packets P 0  and P 2  enables only one intermediate measurement between the terminals  11  and  12 . 
     These lead time calculations are very useful in particular for locating congestion in the network. 
     Analyzing the packets P 0 , P 2  and Pn−1 is a fast way to calculate an average value, a minimum value and a maximum value of the transmission lead time between the terminal  11  and the equipment Ei+1. 
     Similarly, analyzing the packets P 0 , P 1  and Pn−1 is a very fast way to calculate an average value for the transmission lead time between the equipment Ei+1 and the terminal  12 . 
     Over a short period, the production of transmission lead time instantaneous vectors at the lead time of the successive measurements quickly determines complete vectors of the transmission lead times relating to the segments of the path of the stream. 
     Over a long period, and after measuring complete vectors, comparing the complete vectors and the instantaneous vectors is a very fast way to locate the segments whose transmission lead times, and therefore whose performance, worsen if the path is not changed. This also enables comparison of the performance of two different paths. 
     It will be noted that the transmission lead time of a packet is a function of a number of parameters including the size of the packet. In fact, a relatively large packet has a greater transmission lead time than a relatively small packet if the two packets are transmitted in equivalent environments. Accordingly, in order to be able to use instantaneous transmission lead times more accurately, in one embodiment of the invention, an instantaneous time calculated for a given test packet is related to the size of that test packet. 
     From the step  28 , in one embodiment, the collection unit produces packet loss vectors. 
     The table below represents packet losses relating to the various segments of the path of the stream of packets. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 13 
               
               
                   
               
               
                 Losses 
                 T11 
                 . . . 
                 Ei 
                 Ei+1 
                 . . . 
                 T12 
               
               
                   
               
             
            
               
                 P0 
                 0 
                 . . . 
                 0 
                 1 
                 . . . 
                 0 
               
               
                 P1 
                 0 
                 . . . 
                 1 
                 1 
                 . . . 
                 0 
               
               
                 P2 
                 0 
                 . . . 
                 0 
                 1 
                 . . . 
                 1 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 Pn−1 
                 0 
                 . . . 
                 0 
                 0 
                 . . . 
                 0 
               
               
                 TOTAL 
                 0 
                 . . . 
                 1 
                 3 
                 . . . 
                 1 
               
               
                   
               
            
           
         
       
     
     A ‘1’ in the above table means that the packet has been lost or has not been analyzed and a ‘0’ means that the packet has been analyzed. The losses are not counted directly segment by segment. In fact, one or more packets of the same stream may pass in transit through different equipments, especially if load distribution is applied to the path of the stream. Because of this, in order to confirm a packet loss, it is necessary to refer to the packets received at the terminal  12 . 
     The loss vector relating to the packet P 1 , denoted {0, 1, 1, 0}, shows that if a packet is not analyzed in Ei this does not mean that the packet was lost before reaching the equipment Ei. It means that the packet has been lost or was not selected by the analyzer of Ei or has passed in transit through another equipment. For example, Ei and Ei+1 do not know that the packet P 1  exists even though it probably passed through them in transit, since it reached the destination terminal  12 . 
     The loss vector relating to the packet P 2 , denoted {0, 0, 1, 1}, indicates that the packet P 2  was probably lost between Ei and the destination terminal  12 , because it has not been received by the terminal  12  and Ei analyzed it. 
     In certain cases, it is advantageous to introduce a collection unit comprising two modules into the architecture of the network described above. For example, if a first module of a collection unit is adapted to receive and manage stream descriptions sent by the measurement units but not to receive packet stream descriptions from the terminals, a second module is added to receive the packet stream descriptions. 
       FIG. 3  shows a method as claimed in one embodiment of the invention used in an architecture of the above kind. The method comprises the following steps:
         step  302 : sending to the terminal  11  and to the second module  301  a message requesting copying by the first module  300  of the test stream descriptions between the terminals  11  and  12 , corresponding to a measurement start;   step  21 : sending of the sequence of test packets from the terminal  11  to the terminal  12 ;   step  22 ,  22 ′: selection of the test packets and analysis of the stream on the basis of the packets selected by the measurement units of the equipments Ei; in the course of this selection step, each selected test packet is also dated by associating a temporal reference therewith;   step  23 ,  23 ′: analysis of the selected packets and storage in a stream characteristics table entry corresponding to the stream, followed by storage in the fields denoted ‘First’ and ‘Last’ of a passage time reference of the first and last analyzed packets;   step  24 : reception of the sequence of test packets by the terminal  12 ;   step  25 : sending of the packet stream description by the terminal  11  to the first module  300 ;   step  26 ,  26 ′: on end of stream detection (inactivity of stream, for example), exporting of the stream descriptions by the measurement units of the equipments Ei to the second module  301 ;   step  27 : sending of the stream description by the terminal  12  to the first module  300 ;   step  303 : transmission on the fly of the stream descriptions between the terminals  11  and  12  by the second module  301  to the first module  300 ;   step  304 : grouping and correlation of the data relating to the stream by the first module  300 ;   step  305 : production by the first module  300  of the vectors of the information relating to equipments of the path of the stream or to segments of the path taken by the packets of this stream or relating to the complete path.       

     In a configuration in which the collection unit comprises two modules  300  and  301 , the module  300  of the collection unit preferably uses input filtering to filter only the information relating to the streams studied. 
     In one embodiment of the invention, a stream description ticket further comprises a Time To Live field ‘TTL’. Such information is conventionally contained in a field of a stream packet. By exporting this information, the collection unit is therefore able very easily to put the equipments Ei on the path of the stream into order. 
     The present invention proves to be highly advantageous in all fields of packet transmission network metrology, in particular for dimensioning networks, for commissioning and maintaining networks, and for monitoring the quality of service provided. In fact, in the event of detection of “end-to-end” transmission problems, one embodiment of the invention can very quickly locate the segment or segments of the complete path causing the problems, on the basis of very accurate and very reliable measurements. Furthermore, the measurements provided are made available and usable very quickly. Moreover, one embodiment of the invention is very easy to implement in existing networks. 
     It will further be noted that it is easy to deduce from the present description an embodiment of the present invention in which a terminal sends the packet stream to more than one terminal. Accordingly, the measurement method as claimed in one embodiment of the invention may easily be applied to ‘multicast’ transmission to multiple destinations. 
     In a situation of this kind, it is possible for some or all of the terminals receiving the packet stream sent to send a received packet stream description to the collection unit. 
     If the collection unit receives a plurality of received packet stream descriptions it can, for example, deduce therefrom the number of receiver terminals that have actually received the ‘multicast’ stream. 
     In another embodiment of the present invention, the function of the collection unit as previously stated may advantageously be provided by a plurality of entities in a hierarchical architecture. In an architecture of this kind, a collection unit comprises a plurality of collection entities and a central entity. 
     The terminal and the measurement unit are then connected to at least one of the collection entities. 
     For example, a terminal may send packet stream descriptions to one of the collection entities and a measurement unit can send stream descriptions to another collection entity. Each of those collection entities can then forward the descriptions received to the central entity, which identifies each analyzed packet of the stream as a function of the stream description in order to correlate, packet by packet, the stream description and the packet stream description received in this way.