EP1766889A1

EP1766889A1 - Marking of a datagram transmitted over an ip network and transmission of one such datagram

Info

Publication number: EP1766889A1
Application number: EP04805663A
Authority: EP
Inventors: Christophe Proust; Noël CANTENOT
Original assignee: France Telecom SA
Current assignee: Orange SA
Priority date: 2003-12-26
Filing date: 2004-12-08
Publication date: 2007-03-28
Also published as: US20070140265A1; WO2005071902A1

Abstract

The invention relates to a method of marking a datagram (20-22) that is transmitted over an IP-type communication network by routers (4, 5), comprising the inclusion of references in the fields of the datagram (FFIV). According to the invention, when a datagram (20, 22) is received by a router (5), the router reads a reference entered in the datagram and searches for said reference in a reference table stored inside the router. If the read reference is not contained in the reference table, the router selects a new reference from the table. The references can correspond to routes between the router and a datagram receiving terminal. The selection of a route reference can take account of a charge rate value assigned to said route.

Description

_Marked _U P _{_AN DATAGRA} _MRS TRANSMITTED IN IP NETWORK AND TRANSMISSION OF SUCH DATAGRAM

The present invention relates to a method of marking a datagram transmitted in a communication network of the IP (“Internet Protocol”) type. It also relates to a transmission method which can use a marking of a datagram thus carried out. The identification of the path followed by a datagram in a communication network is an important issue for several aspects. This concerns, in particular, certain services intended to guarantee transmission quality guarantees. For example, it is preferable that successive datagrams of the same flow follow the same path in the network, in order to avoid that the order of arrival of the datagrams at the destination terminal is different from the order of sending of the same datagrams by the5 transmitting terminal. A transmission method exists, according to which the path followed by a datagram in the network is registered in the datagram by the terminal transmitting the datagram. This process is known as "source routing". A drawback of this method comes from the fact that the terminals 0 do not know the network topology, that is to say that they do not know the available or unavailable links of the network. The path entered in the datagram is definitively fixed at the time of the transmission of the datagram by the transmitting terminal, and no subsequent modification of the path is possible depending on the possible unavailability of certain links in the network. This transmission method therefore does not make it possible to activate adaptive mechanisms of the routing function in IP networks. In addition, the use of the “source routing” method does not reduce the risk of the appearance of congestion phenomena in the network, that is to say situations according to which the number of datagrams to be routed by a determined link. of the network reaches or exceeds the maximum transmission capacity of this link. Furthermore, in an IPv4 type network, the path registered in a datagram cannot contain more than nine routers, because of the size of the field of the datagram in which this path is registered. Finally, the “source routing” process lacks security, in the sense that the source IP address of the first datagrams can be usurped to cross an access control system based on this address. Second datagrams sent in response to the first datagrams also using the “source routing” process can then be diverted. According to a known method of identifying the path followed by a datagram in a network, called "record routing", each router through which a datagram passes inscribes its IP address in this datagram, following the address entered by the previous router on the path followed by the datagram. A traceability of the path followed is thus obtained, by reading the series of addresses entered in the datagram. This method of identifying the path followed has the disadvantage of not making it possible to orient a datagram at the level of a router as a function of data external to this router at the time of the transmission of the datagram. In addition, in the same way as for the “source routing” method, the number of routers of the path followed by a datagram registered in this datagram according to the “record routing” method is limited to nine routers. An object of the present invention is to develop a marking of a datagram transmitted in a communication network, which does not have the drawbacks of the “source routing” and “record routing” methods indicated above. The invention provides a method of marking a datagram transmitted in a communication network comprising routers connected together by transmission links. The datagram is transmitted from a terminal sending the datagram connected to a first network router to a terminal receiving the datagram connected to a second network router. The datagram comprises a vector formed of ordered fields each containing a reference and a vector index field. In addition, each router in the network has a table of references. The marking process comprises the following steps, when a router receives the datagram: - reading a value in the index field of the datagram, - reading the reference contained in the vector field of the datagram designated by the value d 'index read; - if the router table does not contain the reference read, writing, in the vector field of the datagram designated by the index value read, of a reference selected in the router table; - entry, in the index field of the datagram, of a value equal to the value read incremented by one; and - transmission of the datagram to a next router on the network. According to the method of the invention, the recording of a new reference in the datagram by a router is not systematic. The reference read, which constitutes data external to the router during the transmission of the datagram, is preferred to remain registered in the datagram. The registration by the router of a new reference occurs when the reference read is not contained in the router reference table. The router is then autonomous in selecting, in its reference table, the new reference to be entered in the datagram. When the receiving terminal receives the datagram, the references entered in the vector fields of the datagram are each identical to a reference contained in the reference table of a router having transmitted this datagram. More precisely, the reference registered in the n ^th field of the vector is contained in the table of the n ^th router located on the path followed by the datagram in the network, n being a value taken by the vector index of the datagram during the transmission of it in the network.

Depending on the meaning of the references, various information can thus be collected when the datagram is received by the receiving terminal. When each reference identifies a route along which the datagram was transmitted by a router, knowledge of the meaning of the references in the table of each router makes it possible to reconstruct the path followed by the datagram in the network.

The marking method according to the invention is safe, because the mere reading in the datagram of the references written in the fields of the vector is not sufficient to obtain significant information. It is also essential to know the meaning of the references for each router. Intercepting a datagram without having this knowledge does not allow an exploitation of the references registered in this datagram.

According to the preferred embodiment of a marking method according to the invention, when the datagram belongs to a stream of datagrams successively transmitted by the transmitting terminal to the receiving terminal, the reference read by the router is identical to a registered reference by this router during the transmission of an earlier datagram of said stream. If the network has stable operation, that is to say that no breakdown or congestion of certain network links occurs, which affects a router through which said previous datagram has passed, the same reference contained in the table a router through which several successive datagrams of the flow pass is then associated with all these datagrams. An identical marking is thus obtained along the path followed by the successive datagrams of the flow, to which a specific processing associated with this flow can be attached. The invention also relates to a method of transmitting a datagram in which route references are written. These references may have been previously entered in the datagram using a marking process as described above, but not necessarily. They may also have been entered in the datagram by another method, such as, for example, a “source routing” type method. In this case, a knowledge of the network topology by the terminals is necessary, as well as a knowledge by the terminals of the reference tables of the routers. For the implementation of the transmission method, a network router has a table of references associated with respective routes between this router and a destination terminal of the datagram connected to the network. Preferably, the reference table is associated with a unique destination prefix contained in a routing table of the router. The datagram transmission method then includes the following steps: - when the datagram is received by the router, reading a reference in the datagram; and - search for the reference read from the router reference table,. if the table contains the reference read, transmission of the datagram along the route with which the reference read is associated,. otherwise, selection of a reference in the table, and transmission of the datagram along the route with which the selected reference is associated. A transmission method according to the invention is implemented at the level of the IP protocol layer. A first advantage of such a transmission method lies in the fact that it requires only an adaptation of the IP layer, without modification of the other layers of protocols implemented in the routers. An existing communication network can therefore be easily adapted for the implementation of such a transmission method. A second advantage of a transmission method according to the invention comes from the fact that it allows an initial qualification of the path followed by a datagram in the network. In fact, the transmission of a datagram by a router takes priority into account a route reference read in this datagram. Thus, successive datagrams of the same stream, transmitted by a given transmitting terminal bound for a given receiving terminal, are transmitted in the network along identical routes, if these datagrams contain, during their transmission by the transmitting terminal , with the same references, and if the network has stable operation. However, when the reference table of a router does not contain the reference read from a datagram, this router selects a route from possible routes to transmit the datagram. The transmission method of the invention therefore conforms to the main mode of operation of an IP network, by independent elementary transmissions, designated by "hop by hop" in English. Preferably, the reference selected in the reference table of the router is also written into the datagram using a marking method as described above. When marking and transmission methods according to the invention are simultaneously implemented within an IP network, in addition to the functions of transmitting datagrams and knowing the network topology, the routers can modify the references of registered routes in datagrams. Initial references can be entered in the datagrams by the terminals, without knowledge of the network topology. To this end, the terminals can store references read in a received datagram, in order to use these read references as initial references for an emitted datagram. According to the preferred implementation mode of a transmission method according to the invention, the router reference table further comprises, for each reference of said table, a load rate value assigned to the route to which said reference is associated. The load rate of a route characterizes the amount of traffic carried along this route. When a datagram is transmitted by a router, if the router's reference table does not contain the reference read from the datagram, the selected reference can correspond to a minimum load rate value among the routes with associated references contained in said reference table. Such a transmission method therefore takes into account the load rate of the different routes calculated. This prevents congestion from occurring on particular network transmission links, or at least reduces the risk of congestion occurring. This results in greater stability in the operation of the network: no load oscillation is observed, between different parts of the network. Indeed, such a transmission method tends to distribute and maintain the flow of datagrams in a balanced manner in the network according to the availability of resources. The invention also relates to a terminal adapted to implement a marking process as described first. Such a terminal comprises: - means for producing a datagram intended to be transmitted by the terminal, the datagram comprising a vector of ordered fields and a vector index field; means for registering an initial reference in each field of the datagram vector intended to be transmitted by the terminal; and - means for registering an initial value in the index field of the datagram intended to be transmitted by the terminal. Preferably, the terminal further comprises: - means for reading second references in fields of an additional vector contained in a datagram received by the terminal; and - means for storing, in a table of communication session contexts of said terminal, second references with communication session context data of the received datagram, so that the initial reference entered in each field of the vector of the datagram intended to be sent by the terminal is a so-called second reference read in a field of the additional vector of the received datagram, when the datagram intended to be sent belongs to the communication session of the received datagram. Thus, previously determined route references can be used for the datagram intended to be transmitted. The means for producing the datagram intended to be transmitted can be adapted so that the datagram intended to be transmitted further comprises an additional vector of fields. The terminal then further comprises: - means for reading first references in fields of a vector contained in the received datagram; means for storing, in the table of contexts of communication sessions of said terminal, said first references with the communication session context data of the received datagram; and means for registering said first references in the fields of the additional vector of the datagram intended to be sent by the terminal, when the datagram intended to be sent belongs to the communication session of the received datagram. Thus, when two such terminals each send and receive datagrams of a go flow or a return flow pertaining to the same communication session, initial references can be entered in a datagram of the go flow by the terminal transmitter of this datagragram, which are respectively identical to references contained in fields of said additional vector of a datagram of the return flow. By applying this mechanism to the two transmitting and receiving terminals of a datagram, it follows that the reference read by a router in this datagram is identical to a reference written by this router during the transmission of an earlier datagram of this stream. The invention further relates to a router suitable for implementing a transmission method as described secondly. Such a router comprises: means for reading a value in a vector index field of a datagram received by the router; means for reading a reference contained in a vector field of said datagram designated by the index value read; - means for storing a reference table; - means for associating the references contained in the table with respective routes; means for searching for a read reference, in the reference table of said router, arranged to control a transmission of said datagram along the route with which the read reference is associated, if the reference table contains the read reference; means for selecting a reference in the reference table, arranged to be activated if the reference table does not contain the reference read, and arranged to control a transmission of said reference datagram along the route to which the selected reference is associated; and means for recording, in the index field of said datagram, a value equal to the value read incremented by one. According to an advantageous embodiment of such a router, the association means are included in means for calculating a routing table of this router. In addition, the calculation means belong to a control unit of the router. The router can be adapted to simultaneously implement a method of marking a datagram passing through this router, as described first. For this, it further comprises means for writing the reference selected in the vector field of the datagram designated by the index value read. Even if the combination within the same router of the implementation of the marking and transmission methods which are the subject of the invention is particularly advantageous for obtaining an efficient operation of the network, the router may include means for implementing the method regardless of the presence, in this same router, of datagram marking means. And vice versa. For a router which combines implementations of the two methods, some of the means mentioned above for each method may be common to the two methods. The invention finally relates to a communication network which comprises a router as described above. Other features and advantages of the present invention will appear in the description below of an example of non-limiting implementation, with reference to the appended drawings, in which: - Figure 1 shows a communication network in which the invention can be implemented; - Figure 2 illustrates the structure of an IP datagram header as used for an implementation of the invention; - Figures 3a and 3b, intended to be associated, illustrate different stages of a transmission according to the invention of several datagrams, within a network as shown in Figure 1; - Figure 4 is a flow diagram of a method of transmitting a datagram according to the invention. According to FIG. 1, a communication network by transmission of datagrams 100, of the IP type, comprises routers 4-9 linked together by transmission links. Thus a chain of routers forms a route in the network 100, to transmit a datagram. Each router can include a routing unit (or

“Forwarding unit”, which transfers the datagrams between two links linked to this unit, and a command unit (or “control unit”) which supervises the activity of the routing unit. For router 5, the routing unit and the control unit are respectively referenced 5a and 5b. Terminals are connected to some of the routers in network 100.

These terminals can be of different types, such as, for example, computer units, mobile communication units, etc. In FIG. 1, computer units 1 and 10 are connected to routers 4 and 7, respectively. Data produced by terminal 1 and intended for terminal 10 is distributed by terminal 1 in IP datagrams. These datagrams are transmitted by some of the routers of the network 100 to the terminal 10. It is considered below that the operation of an IP type network is known. In this context, in FIG. 1, each transmission link of the network 100 is identified by a prefix DP, followed by identifiers of each of the routers or terminals connected to this link. Each IP datagram has a header as shown in Figure 2, for version 4 of the IP protocol (IPv4). The header includes a basic header part Bl present in all the datagrams, and an optional header part B11. The header part B1 has a fixed length of 20 bytes. The header part B11 can have a variable length. Among the fields of the header part B1 appear the field "SOURCE ADDRESS", in which the IP address of the terminal sending the datagram is indicated, and the “DESTINATION ADDRESS” field, in which the IP address of the terminal receiving the datagram is indicated. Other fields include: - the “Type Of Service”, or TOS, field in which the way in which the datagram is to be managed is specified; - the "TOTAL LENGTH" field, in which the total length of the datagram is specified; - the "ID" field intended to receive an identification number of the datagram, in particular with a view to possible fragmentation of the datagram; - the “TTL” or “TIME TO LIVE” field, in English, in which is indicated a maximum duration of existence of the datagram being routed in the network; and - the “PROTOCOL” field, in which the high-level protocol to which the data placed by the sending terminal in the datagram data field (or “payload”, in English, and not shown in FIG. 2) is indicated. ). The header part B11 comprises a first field denoted “OPT. TYPE ", 1 byte. This field is intended to receive a reference constructed according to a nomenclature known to those skilled in the art. This reference specifies the presence, the class and a dedicated option number, which make it possible to identify the nature of the content of the header part Bll. By way of example, for the embodiment of the invention described below, this reference can be 10011001 in binary representation of the byte. The length of the header part B11, expressed in bytes, is indicated in the "OPT." LENGTH ”(1 byte). For the implementation of the invention described here, the maximum possible length for the header part B11 is used, namely 40 bytes. The 38 bytes remaining available in the header part B11 are divided into four fields arranged as follows: - a first field denoted FVI, for “Forward Vector Index” in English, of 1 byte, intended to receive a value digital index; - a second field denoted BVL, for “Backward Vector Length”, also of 1 byte, intended to receive a value of vector length; - a first ordered series of 36 fields, noted FFIV, for "Forward Flow Identifier Vector", of 18 bytes in total. The fields of this first series, arranged one after the other, form a first vector and are intended to receive 36 first digital references, coded on 4 bits each. For the sake of clarity, the index and the first vector are respectively designated below by the FVI index and the FFIV vector. The FVI index is used to locate a field in the FFIV vector, by the sequence number of the position of this field in the FFIV vector, counted from the start of the vector. Thus, the value of the FVI index varies between 1 and 36. - a second ordered series of fields, denoted BFIV, for "Backward Flow Identifier Vector", also of 18 bytes in total. This second series of fields is organized in the same way as the first series of fields. The BFIV fields form a second vector and are intended to receive 36 second digital references coded on 4 bits each. This second vector, which corresponds to the additional vector mentioned in the general description of the invention, is designated hereinafter by BFIV vector. It is understood that the positions of the fields FVI and BVL, as well as those of the fields of the vectors FFIV and BFIV shown in FIG. 2 are given by way of example. Other positions can be chosen, in order to obtain alternative embodiments of the invention. Such alternative modes of implementation are also included in the invention. However, it is particularly advantageous, in the context of the IPv4 version of the IP protocol, to design the FVI and BVL fields, as well as the FFIV and BFIV vector fields, so as to exploit the maximum length of the part of- Bll head. It is also understood that the invention can be implemented in an equivalent manner within the framework of version 6 of the IP protocol (IPv6), by adapting the fields FVI and BVL, as well as the fields of vectors FFIV and BFIV defined above. above to the specifics of this latest version of the IP protocol. In particular, the IPv6 version allows the adoption of longer field and vector lengths, thanks to the possibility of defining an option header for size larger than 40 bytes. Each router of network 100 has a routing table used by the routing unit of this router to transmit the received datagrams. This routing table is designated by FIB table, for "Forwarding Information dataBase" in English. Table 1 below represents a part of such a routing table, established by way of example for router 5 in FIG. 1:

Table 1

The FIB table must be read in rows, each row corresponding to a route. For example, the second line characterizes the direction to take, that is to say the IP address of the following router, ie DP_5_6: 6, to reach the destination prefix DP_7_10. The first column of the FIB table, entitled "Destination Prefix" in English, groups DP destination prefixes. As an example, the destination prefix noted DP_7_10 corresponds to the interconnection subnet between router 7 and terminal 10. The second column of the FIB table, entitled "Next Hop", indicates an IP address of a router or terminal connected to router 5 by a single transmission link, for each destination prefix indicated in the first column. IP addresses are constructed in a known manner, with a DP prefix followed by an identifier of the terminal or the router. The columns “Interface” and “Encapsulation L2” give useful information for the transmission of the datagram, without relation to the invention. When a datagram is received by a router, the IP address of the terminal receiving this datagram is read in the "DESTINATION ADDRESS" field in the header part B1 of the datagram. In a known manner, the destination prefix of the address read is isolated using a network mask, and is identified with one of the destination prefixes contained in the first column of the FIB table according to a principle of greater coincidence, or "longest match" in English. According to the invention, the FIB table of each router is completed in the following manner, illustrated by table 2 below. Table 2 also represents a part of the FIB table, established by way of example for router 5 (see FIG. 1):

Table 2

Compared to a FIB table conforming to table 1, a FIB table conforming to table 2 has two additional columns. The first column contains route references, and is titled "Ref." The second column, titled "Load", contains load rate values. A FIB table conforming to table 1 indicates a single possibility of transmission of a datagram by the router, for each destination prefix. Unlike, an FIB table conforming to table 2 indicates a or several possibilities of transmitting a datagram for each destination prefix. Each possibility corresponds to a different route between the router concerned, that is to say the router 5 in the example considered, and the destination terminal whose IP address is read in the datagram. The “Ref.” Column associates a numerical reference to each of these routes. These references distinguish the different routes taken into account in the FIB table, which correspond to the same destination prefix. The same reference can possibly be used several times in the FIB table, to designate routes corresponding to different destination prefixes. For the same destination prefix, the references associated with the routes taken into account in the FIB table have only an identification function, and do not correspond to a classification. The references used depend on the modifications which have occurred in the FIB table during successive updates thereof, established by the control unit of the router. The reference table mentioned in the general description of the invention is formed by all of the rows of the FIB table which correspond to a single determined destination prefix. Thus, in the example corresponding to FIG. 1 and to Table 2, the reference table considered for a datagram whose destination prefix is DP_7_10 comprises the third to fifth rows of the FIB table, without counting the row of column names . For each row of the FIB table, ie for each route taken into account, the "Load" column indicates a value of load rate. This value is determined by the control unit based on network status data. Such data are, for example, load rate values of at least some of the links in network 100. The value indicated in the “Load” column for each row of the FIB table can be, for example, at the most large of the load rate values of all links along the route corresponding to this line. It can be expressed in various ways, such as, in particular, as a percentage of a maximum transmission capacity by this route, or bandwidth. We will now describe in detail transmissions of several datagrams within the network 100 represented in FIG. 1, with reference to FIGS. 3a and 3b. The letters A to O indicated in Figures 3a and 3b identify different stages of these transmissions. When data is produced by the terminal 1 intended for the terminal 10, the terminal 1 distributes this data in successive datagrams. The process for creating datagrams is considered to be known. In particular, the terminal 1 creates a communication session context (step A in FIG. 3a). This communication session context includes in particular the following data: the respective IP addresses of the terminals 1 and 10, a transport protocol number (for example the TCP or UDP protocols) and source and destination port numbers. The data of the communication session context are stored in the terminal 1. When creating a first datagram 20, the terminal 1 configures the header part B1 of this datagram. In particular, it registers the respective IP addresses of terminals 1 and 10 in the "SOURCE ADDRESS" and "DESTINATION ADDRESS" fields. It also configures the header part Bll, by entering the following initial values in the different fields: - OPT field. TYPE: 10011001 - OPT field. LENGTH: 40 - FVI field: 1 - BVL field: 0 - FFIV vector fields: 0, 0 0 - BFIV vector fields: 0, 0 0 According to the method given here as an example, 1 is the initial value of the index FVI, and 0 is the initial value entered in the BVL field. 0 is also the initial reference entered in all the fields of the vectors FFIV and BFIV. The reference 0 is reserved for the initialization function: it is not used in the router reference tables to identify a route. As will appear below, the initialization at 0 of the fields of the vectors FFIV and BFIV of a datagram by a terminal means that this datagram is either an isolated datagram, or a first datagram of a flow, or a datagram subsequent of a flow for which the transmitting terminal does not have predetermined initial references. These references entered in the fields of the vectors FFIV and BFIV of the datagram 20, as well as the values written in the fields FVI and BVL of the datagram 20, are stored by the terminal 1 with the data of the context of the communication session. The terminal 1 then transmits the datagram 20 to the router 4 (step B).

It is assumed that the router 4 in turn transmits the datagram 20 to the router 5 (step C). During this transmission, the router 4 writes the value 2 in the index field FVI of the datagram 20, as well as the reference 3, given by way of example, in the first field of the vector FFIV of the datagram 20. The method of transmission of the datagram 20, implemented within each router, is now described in detail for the router 5. During this process, the router 5 selects a line in its FIB table (table 2). This selection is made in two successive stages. During a first step of selecting a line from the FIB table, the router 5 reads the IP address of the destination terminal of the datagram 20 in the "DESTINATION ADDRESS" field of the header part Bl. isolates the destination prefix from this address, and compares it to the destination prefixes contained in the first column of the FIB table. According to the process of greatest coincidence (“longest match”), it selects from the FIB table the longest destination prefix which corresponds to that of the IP address of the destination terminal. In the example described here, the selected destination prefix is noted DP_7_10 (table 2). The references contained in the “Ref.” Column of the FIB table to the lines corresponding to the destination prefix DP_7_10 form the reference table taken into account when transmitting datagram 20. This reference table is therefore a part of the FIB table which corresponds to the selected unique destination prefix DP_7_10. The second step of selecting a row from the FIB table is now described with reference to FIG. 4. The various steps mentioned in FIG. 4 are carried out in the routing unit 5a. Router 5 first reads the value entered in the FVI index field of datagram 20 (step 30). The value read is 2. It then reads the reference contained in the field of the vector FFIV whose position within this vector corresponds to the value read in the FVI field (step 31). The reference 0 is thus read in the second field of the vector FFIV. Since the reference read is equal to the initialization reference (step 32a), the router 5 is then autonomous in determining the route along which it transmits the datagram. It selects, in the reference table corresponding to the destination prefix DP_7_10, the reference which corresponds to a minimum charge rate value (step 33). In the case of the FIB table of table 2, the reference 4 corresponds to the minimum charge rate value. The router 5 then registers the reference 4 in the field of the vector FFIV designated by the index FVI (second field of the vector FFIV - step 34 of FIG. 4). It then increments the value of the FVI index by one and writes the value obtained in the FVI field of the datagram 20 (step 35). The datagram 20 then contains the values and references indicated in FIG. 3a, for step D. The line of the FIB table which corresponds to the selected destination prefix and to the reference 4 is the selected line. Router 5 transmits datagram 20 according to the indication in the “Next Hop” column for the selected line. In the example described, it transmits the datagram 20 to the router 6. The routers 6 (step E of FIG. 3b) then 7 (step F) each transmit the datagram 20 by following a process identical to that described for the router 5. The terminal 10 then receives the datagram 20 containing the index value FVI and the references, written in the fields of the vectors FFIV and BFIV, indicated in step G, by way of example. The terminal 10 stores the data of the context of the communication session of the datagram 20, read in the header part B1 of the datagram 20. It also stores, with these data, the contents of the fields FVI and BVL, as well as those FFIV and BFIV vector fields. The terminal 10 then reads the data contained in the data field (or “payload”) of the datagram 20. It is now assumed that the terminal 10 in turn produces data intended for the terminal 1, in response to the data carried by the datagram 20. The data produced by terminal 10 are transmitted in the context of the same communication session as that of the datagram 20. The datagram 20 therefore belongs to a go flow coming from this communication session, and new datagrams sent by the terminal 10 belong to a return flow from this session Communication. The terminal 10 creates a new datagram 21, by taking the data from the communication session context of the datagram 20 (step H of FIG. 3b). The datagram 21 has a structure identical to that of the datagram 20. The terminal 10 completes the fields of the header part B1 of the datagram 21 as described above for the terminal 1 and the datagram 20, by exchanging the data relating to the sending and receiving terminals. In addition, the terminal 10 retrieves, with the data of the stored communication session context, the references and values memorized during the reception of the datagram 20. It writes them in the fields of the header part Bll of the datagram 21 , as follows: - in the successive fields of the FFIV vector of the datagram 21: the references contained in the fields of the BFIV vector of the datagram 20, in the same order; - in the successive fields of the BFIV vector of the datagram 21: the references contained in the fields of the FFIV vector of the datagram 20, in the same order; and - in the BVL field of the datagram 21: the value read in the FVI field of the datagram 20. Like the terminal 1 during the transmission of the datagram 20, the terminal 10 writes the initial value 1 in the index field FVI of datagram 21 (step I). The transmission of the datagram 21 in the network 100 to the terminal 1 is carried out in a manner analogous to the transmission of the datagram 20 described above. It should be noted that the datagram 21 does not pass, a priori, by the same routers as the datagram 20, and that the numbers of routers through which each of the datagrams 20 and 21 respectively pass are a priori different. In FIGS. 3a and 3b, there is the number of routers through which the datagram 21 passes. The contents of the BVL field and BFIV vector fields of the datagram 21 are transported during the transmission of the datagram 21 in the network 100, without being used. The reception of the datagram 21 by the terminal 1 (step J of FIG. 3a) is carried out in an identical manner to the reception of the datagram 20 by the terminal 10, described above. In particular, the contents of the fields FVI and BVL, as well as those of the vectors FFIV and BFIV of the datagram 21, are stored by the terminal 1 with the data of the context of the communication session. The references entered in the BFIV vector fields of the datagram 21 of the return flow, and the value entered in the BVL field of the datagram 21, are then stored in each of the two terminals 1 and 10. Thus, the references entered by the routers in the FFIV vector fields of the datagram 20, as well as the last value entered in the FVI index field of the datagram 20, ie the value entered by the router 7, are stored in the two terminals 1 and 10 with the data of the context of the communication session. The method now described applies to the situation where the terminal 1 produces new data intended for the terminal 10, in response to the data conveyed by the datagram 21. The terminal 1 then creates a datagram 22, according to the procedure already described, by taking the data from the stored communication session context (step K of FIG. 3a). The datagram 22 belongs to the same go flow pertaining to this communication session as the datagram 20. The terminal 1 enters, in the FVI field of the datagram 22, the initial value 1, and, in the BVL field of the datagram 22, the value y of the FVI index read in the datagram 21 upon reception of the latter by the terminal 1. It also writes, in the fields of the vector FFIV of the datagram 22, the references read in the fields of the vector BFIV of the datagram 21. From even, it inscribes in the fields of the vector BFIV of the datagram 22, the references read in the fields of the vector FFIV of the datagram 21. The values and references thus obtained for the fields of the datagram 22 are indicated in step L of FIG. 3a . In other words, when a datagram belongs to a go stream of datagrams successively transmitted by a transmitting terminal to a receiving terminal, said go flow coming from a communication session, the fields of the BFIV vector of said datagram are intended to receive references entered in the fields of a FFIV vector of a datagram of a return flow pertaining to said communication session, transmitted by the terminal receiving the datagrams of the outward flow, and received by the terminal transmitting the datagrams of the flow- go before the transmission of said go flow datagram. Likewise, the BVL field of said go flow datagram is intended to receive the last value entered in the FVI field of said return flow datagram. It is assumed that the datagram 22 is transmitted to the router 4 by the terminal 1, then to the router 5 by the router 4 (step M), in the same way as the datagram 20. When the datagram 22 is transmitted by the router 4, that -ci increments the value of the FVI index to 2. The router 4 also leaves the reference 3 written in the first field of the vector FFIV of the datagram 22. The description of the method of transmission of a datagram by a router of the network 100 is now completed, in the case of the datagram 22 transmitted by the router 5. The first step of selecting a line from the FIB table is identical to that described for the transmission of datagram 20. During the second step of selecting a line from the FIB table, router 5 analyzes the FFIV vector of the datagram 22 according to the method of FIG. 4. It reads the reference indicated by the FVI index (steps 30 and 31). The reference read is 4, in accordance with the entry made by the router 5 in the datagram 20, during step D of the transmission of the latter. This reference read being different from the initialization reference 0 of the vector fields (step 32a), the router 5 examines whether the reference table determined during the first step of selecting a line of the FIB table contains the reference 4 (step 32b). Two situations can then arise: according to a first situation, the reference table still contains reference 4, and according to a second situation, it no longer contains reference 4. The first situation occurs, in particular, when the FIB table of router 5 has not been modified between the respective transmissions of datagrams 20 and 22. It intervenes also if the reference 4 has been maintained in the FIB table for the destination prefix DP_7_10 during updates of the FIB table which have occurred between the transmissions of datagrams 20 and 22. In this case, in accordance with the diagram in FIG. 4, the FFIV vector is not modified by router 5, the FVI index is incremented (step 35), and router 5 transmits datagram 22 to router 6 (step 36). The transmissions of datagrams 20 and 22 by the router 5 are then identical. The second situation occurs when an update of the FIB table of router 5 has occurred between the transmission of datagram 20 and that of datagram 22. It is assumed, for the example, that the update did not consist, with regard to the routes connecting the router 5 to the terminal 10, only with the deletion of the line of table 2 corresponding to the destination prefix DP_7_10 and the reference 4. At the end of the first step of selecting a line of the FIB table, the router 5 has selected in its FIB table the two lines corresponding to the destination prefix DP_7_10. These two lines constitute the new reference table considered. The first of these two lines corresponds to the route reference 1, and the charge rate value indicated is 70%. The second of these two lines corresponds to the route reference 6, and the charge rate value indicated is 20%. The router 5 selects one of these two references for which the route has the minimum load rate value (steps 32b and 33): the reference 6. It then writes this reference in the second field of the vector FFIV of the datagram 22 (step 34 ). In the same way as previously, it increments the FVI index of the datagram 22 (step 35). Finally, it transmits the datagram 22 according to the indication in the “Next Hop” column of the FIB table for the line corresponding to the destination prefix DP_7_10 and to the reference 6 (step 36). In the present example, the router 5 transmits the datagram 22 to the router 9. The steps N and O of FIGS. 3a and 3b illustrate the second situation which has just been described, x is a reference entered by the router 9. On receipt of the datagram 22 by the terminal 10, the terminal 10 updates the values and references stored with the data of the context of the communication session. At the end of this update, the references entered in the fields of the vector FFIV of the datagram 21, copied into the fields of the vector BFIV of the datagram 22 by the terminal 1, are stored in the two terminals 1 and 10. The same is true for even for the last value entered in the FVI index field of the datagram 21. It appears from the above description that the combination of the marking and transmission methods of a datagram according to the invention provides the following advantages: when a new datagram belongs to a flow for which datagrams have already been transmitted, the use of a route followed by an earlier datagram of the same flow is preferred for the new datagram; - when the route used for the previous datagram of the stream is no longer available, the new datagram is transmitted according to a new route; and the new route is determined so as to distribute the datagram flows between different transmission links of the network according to their degrees of availability. This association combines transmission by independent elementary jumps (“hop by hop”) and a reduction in the risk of network congestion. Stable network operation results. It is understood that variants can be introduced with respect to the mode of implementation of the invention which has been described above. In particular, in FIG. 4, steps 32a and 32b can be grouped together in a single step, corresponding to the same test as that of step 32b. Finally, in particular embodiments of the invention, references written in a field of the vector FFIV or vector BFIV of a datagram, as well as a value written in the field FVI or BVL of a datagram , can only be stored in one of the two terminals sender or recipient of this datagram. Such an implementation may, in particular, relate to a datagram belonging to a return flow.

Claims

1. Method for marking a datagram (20, 22) transmitted in a communication network (100) comprising routers (4-9) connected to each other by transmission links, from a terminal transmitting the datagram ( 1) connected to a first network router (4) to a terminal receiving the datagram (10) connected to a second network router (7), the datagram comprising a vector (FFIV) formed of ordered fields each containing a reference , the datagram further comprising a vector index field (FVI), each router having a reference table, the method comprising the following steps, when a router (5) receives the datagram: - reading a value in the index field (FVI) of the datagram, - reading of the reference contained in the vector field of the datagram (FFIV) designated by the index value read; - if the router table does not contain the reference read, writing, in the vector field of the datagram designated by the index value read, of a reference selected in the router table; - entry, in the index field of the datagram, of a value equal to the value read incremented by one; and - transmission of the datagram to a next router on the network.

2. The marking method according to claim 1, according to which the vector fields (FFIV) and the index field (FVI) are arranged in the header of the datagram.

3. The marking method according to claim 1 or 2, according to which the references contained in the router table (5) are associated with respective routes in the network (100).

4. Marking method according to claim 3, according to which the router reference table (5) is a part of a routing table of said router. router, said part corresponding to a unique destination prefix contained in the routing table.

5. A marking method according to any one of claims 1 to 4, according to which the datagram (22) belongs to a stream of datagrams successively transmitted by the transmitting terminal (1) to the receiving terminal (10), and according to which the reference read is identical to a reference entered by said router (5) during the transmission of an earlier datagram of said stream (20).

6. A marking method according to claim 5, according to which an initial reference is entered in each field of the vector (FFIV) of a first datagram of the stream (20) by the transmitting terminal (1), the initial reference not corresponding to any reference contained in the router reference tables.

7. A marking method according to any one of the preceding claims, according to which the datagram (22) belongs to a go stream of datagrams successively transmitted by the transmitting terminal (1) to the receiving terminal (10), said go flow falling a communication session, and according to which said datagram (22) further comprises an additional vector (BFIV) formed of fields intended to receive references inscribed in the fields of a vector (FFIV) of a datagram of a return-flow (21) pertaining to said communication session, transmitted by the terminal receiving the datagrams of the forward flow (10), and received by the terminal transmitting the datagrams of the forward flow (1) before the transmission of said datagram of the flow -Go (22).

8. The marking method according to claim 7, according to which the references written in the vector fields (FFIV) of the datagram of the return flow (21) are stored with context data of the communication session in at least one. of the two terminals (1, 10).

9. The marking method according to claim 7 or 8, according to which the fields of the additional vector (BFIV) are arranged in the header of said go-stream datagram (22).

10. A marking method according to any one of claims 7 to 9, according to which initial references are entered by the transmitting terminal (1) in the vector fields (FFIV) of said datagram of the go-stream (22), said references initials being respectively identical to references contained in fields of an additional vector (BFIV) of the datagram of the return flow (21).

11. The marking method according to claim 10, according to which the references written in the fields of the additional vector (BFIV) of the datagram of the return flow (21) are stored with context data of the communication session in one to the less than the two terminals (1, 10).

12. A marking method according to any one of claims 7 to 11, according to which said go flow datagram (22) further comprises a vector length field (BVL) intended to receive the last value entered in the field d 'index (FVI) of the datagram of the return flow (21).

13. The marking method according to claim 12, according to which the last value entered in the index field (FVI) of the datagram of the return flow (21) is stored with context data of the communication session in one at least two terminals (1, 10).

14. The marking method according to claim 12 or 13, according to which the vector length field (BVL) is arranged in the header of said go flow datagram (22).

15. The marking method according to claim 12, according to which a value written in a vector length field (BVL) of the datagram of the return flow (21) is stored with context data of the session. communication in at least one of the two terminals (1, 10).

16. Method for transmitting a datagram (20, 22) by a router (5) of a communication network (100), the router having a table of references associated with respective routes between said router and a terminal , recipient of the datagram (10) connected to the network, the transmission method comprising the following steps: - when the datagram is received by the router, reading a reference in the datagram; and - search for the reference read from the router reference table,. if the table contains the reference read, transmission of the datagram along the route with which the reference read is associated,. otherwise, selection of a reference in the table, and transmission of the datagram along the route with which the selected reference is associated; method according to which the reference read has been previously registered in the datagram using a marking method according to any one of claims 1 to 15.

17. The transmission method according to claim 16, according to which the reference selected in the reference table of the router is also written in said datagram (20, 22) using a marking method according to any one of claims 1 to 15. .

18. The transmission method according to claim 16 or 17, according to which the reference table is associated with a unique destination prefix contained in a routing table of said router (5).

19. The transmission method according to claim 18, comprising the following steps carried out upon reception of the datagram by the router

(5) before searching for the reference read from the reference table of said router: - reading of a destination address in the datagram; and - selection in the routing table of said router of the longest destination prefix corresponding to the destination address read, the reference table of said router in which the reference read in the datagram is then sought being associated with the selected destination prefix.

20. Transmission method according to any one of claims 16 to 19, according to which the reference table further comprises, for each reference of said table, a load rate value assigned to the route with which said reference is associated, and according to which the selected reference corresponds to a minimum load rate value among the routes to which are associated references contained in said reference table.

21. Terminal (1, 10) comprising: - means for producing a datagram intended to be transmitted by the terminal (20-22), the datagram comprising an ordered field vector (FFIV) and an index field of vector (FVI); - means for registering an initial reference in each vector field (FFIV) of the datagram intended to be transmitted by the terminal; and means for recording an initial value in the index field (FVI) of the datagram intended to be transmitted by the terminal.

22. Terminal according to claim 21, further comprising: - means for reading second references in fields of an additional vector (BFIV) contained in a datagram received (21) by the terminal; and - means for storing, in a table of communication session contexts of said terminal, second references with communication session context data of the received datagram, in which the initial reference entered in each vector field (FFIV) of the datagram intended to be transmitted by the terminal (21, 22) is a so-called second reference read in a field of the additional vector (BFIV) of the received datagram (20, 21), when the datagram intended to be sent belongs to the communication session of the received datagram.

23. Terminal according to claim 22, in which the means for producing the datagram intended to be transmitted (21, 22) are adapted so that the datagram intended to be transmitted further comprises an additional vector (BFIV) of fields, the terminal further comprising: - means for reading first references in fields of a vector (FFIV) contained in the received datagram (20, 21); means for storing, in the table of contexts of communication sessions of said terminal, said first references with the data of context of communication session of the received datagram; and - means for registering said first references in the fields of the additional vector (BFIV) of the datagram intended to be sent by the terminal (21, 22), when the datagram intended to be sent belongs to the communication session of the received datagram .

24. The terminal as claimed in claim 23, in which the means for producing the datagram intended to be sent (21, 22) are also adapted so that the datagram intended to be sent further comprises a vector length field (BVL), the terminal further comprising: - means for reading a value in a vector index field (FVI) contained in the received datagram (20, 21); means of storing, in the table of contexts of communication sessions of said terminal, the index value read with the data of context of communication session of the received datagram; and - means for registering the index value read in the vector length field (BVL) of the datagram intended to be transmitted by the terminal (21, 22), when the datagram intended to be transmitted belongs to the session communication of the datagram received.

25. Router (5) comprising: - means for reading a value in a vector index field (FVI) of a datagram received (20, 22) by the router; means for reading a reference contained in a vector field (FFIV) of said datagram designated by the index value read; - means for storing a reference table; - means for associating the references contained in the table with respective routes; means of searching for a read reference, in the reference table of said router, arranged to control a transmission of said datagram along the route with which the read reference is associated, if the reference table contains the read reference; means for selecting a reference in the reference table, arranged to be activated if the reference table does not contain the reference read, and arranged to control a transmission of said datagram along the route to which the reference is associated selected; and - means for recording, in the index field of said datagram (FVI), a value equal to the value read incremented by one.

26. A router according to claim 25, further comprising means for writing the reference selected in the vector field (FFIV) of said datagram designated by the index value read.

27. Router according to claim 25 or 26, in which the association means are included in means for calculating a routing table of said router, said means for calculating belonging to a control unit of said router (5b).

28. Router according to claim 27, in which the association means are further arranged for associating a reference table with a unique destination prefix contained in the routing table of said router (5).

29. Router according to claim 28, in which the reference table of said router comprises, for each reference in said table, a load rate value assigned to the route to which said reference is associated, and in which the means for selecting d 'a reference are arranged to select the reference for which the route corresponds to a minimum load rate value.

30. Communication network comprising a router (5) according to any one of claims 25 to 29.