Abstract:
For transporting packets between an access interface of a subscriber installation and a concentrating router of a shared network the access interface carries out control operations on streams of packets transmitted to the concentrating router, within the framework of a contract between the subscriber and a manager of the shared network. After having carried out the control operations concerning a packet to be transmitted, the access interface transmits this packet to the concentrating router with a signature based on a secret shared with the concentrating router, authenticating that the packet has been subjected to the control operations.

Description:
The present invention relates to packet based transmission networks. It applies in particular, but not exclusively, to shared networks operating according to the Internet protocol (IP). 
   The implementation of the invention comes within the framework of contractual relations between a provider of access to the shared network and his customers. The provider is furnished, for the attachment of the installations of his customers, with one or more concentrating routers for the shared network. Transmission lines link this concentrating router to the access interfaces of the customers&#39; installations, which may be private network access router interfaces. 
   Here, the expression “police” functions designates various processing or control operations performed at the level of an interface of the network on data streams which pass through it. By way of nonlimiting examples, mention may be made of the counting of the packets exchanged between a given source address and a given destination address, the allocating of priorities to certain packets, address translations, the selective destruction of certain packets, etc. 
   These police functions may be included within a contractual framework between a subscriber (customer) and a manager of the network (provider of services). Such may for example be the case with functions relating to billing, to flow control, to authorization for access to certain sites linked to the network, to the implementing of reservation protocols such as RSVP, etc. They may also be included within the framework of the internal organization of a public or private network, for example to control certain accesses. 
   Customarily, the police functions pertaining to the contractual framework between the access provider and his customers are implemented at the level of the concentrating router&#39;s attachment interfaces. This router hosts software for controlling the streams which travel around its various interfaces. The packets having certain originating or destination addresses or ports are counted, filtered, rearranged etc. according to the type of service offered. Owing to the large number of installations which may be linked to the concentrating router and to the variety of services which may be rendered in respect of these installations, the various stream controls to be applied may considerably increase the complexity of the router. This drawback is all the more noticeable as more and more diverse processing operations are requested by customers or required by new reservation protocols. 
   Moreover, this organization is not flexible for the customer who wishes to tailor certain characteristics of the service offered to him. To do this he must turn to his provider so that the latter may make the changes required at the level of his concentrating router. 
   An aim of the present invention is to propose a mode of operation of the network which enables a wide diversity of stream controls to be taken into account without resulting in an excessive increase in the complexity of the concentrating routers, and with a relative flexibility of configuration. 
   The invention thus proposes a method of transporting packets between an access interface of a subscriber installation and a concentrating router of a shared network, in which the access interface carries out control operations on streams of packets transmitted to the concentrating router, within the framework of a contract between the subscriber and a manager of the shared network. After having carried out the control operations concerning a packet to be transmitted, the access interface transmits this packet to the concentrating router with a signature based on a secret shared with the concentrating router, authenticating that the packet has been subjected to the control operations. 
   Preferably, the obtaining of the signature and certain at least of the control operations are carried out within one and the same integrated circuit, without physical access immediately upstream of the obtaining of the signature. 
   The stream controls pertaining to the contractual framework between the manager of the network and the subscriber are thus decentralized, thereby avoiding the need for the concentrating router to take on all the diversity of the operations demanded by the various subscriptions. The mechanism for signing the packets guarantees to the manager of the network that the subscriber, who is furnished with the access interface at his premises, does not send him packets which have not been subjected to the stream control operations, that is to say which have sidestepped the police and billing functions. 
   The method gives rise to a distributed architecture of access and of concentration, which is well suited to taking account of the increases in traffic and in diversity of services which future applications will entail. 
   The subscriber benefits moreover from greater flexibility for dynamically defining the characteristics of his subscription. He merely needs to intervene at the level of the access interface with which he is furnished. He may moreover define the police functions pertaining to the contractual framework with the access provider on the same platform as the other police functions which he uses for the internal organization of his installation, thereby simplifying organization thereof. 
   Another aspect of the present invention concerns an access interface for linking an access router of a subscriber installation to a concentrating router of a shared network, comprising means for controlling streams of packets transmitted to the concentrating router, within the framework of a contract between the subscriber and a manager of the shared network, and signature means receiving the packets delivered by the stream control means and producing signed packets transmitted to the concentrating router, each signed packet comprising a signature based on a secret shared with the concentrating router, authenticating that the packet has been subjected to the stream control means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will become apparent in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which: 
       FIG. 1  is a diagram of a network where the invention may be implemented; 
       FIG. 2  is a schematic diagram of an access router of a private installation of this network; 
       FIG. 3  is a schematic diagram of a stream processing device forming part of an interface of the router of  FIG. 2 ; and 
       FIG. 4  is a graph of elementary processing operations undertaken by the device of  FIG. 3 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a wide area shared network (WAN)  10  comprising a certain number of interconnected routers and switches  11 ,  12 . The case where the shared network  10  operates according to the IP protocol is considered here. A certain number of the routers are concentrating routers  12  to which private installations  13  are linked. 
   A private subscriber installation  13  is typically linked to the shared network  10  by means of an access router  15 , one of whose interfaces  16  is linked to a line  17  for transmission from and to the concentrating router  12 . The access router  15  can be linked to other routers of the private installation  13  or to servers or terminals  18  of this installation, by means of other interfaces, which are not represented in  FIG. 1 . 
     FIG. 2  shows an exemplary architecture of the access router  15 . The outside interface  16 , and also the interfaces  20 ,  21  with the remainder of the private installation  13 , are linked to the core of the router consisting of a packet forwarding engine  22 . The forwarding engine  22  forwards the packets from one interface to another on the basis of the address fields and port fields contained in the headers of the packets in accordance with the IP protocol and with any extensions thereof (TCP, UDP, etc), by referring to routing tables. 
   Certain of the interfaces of the access router  15  are provided, in just one or in both directions of transmission, with processing devices, or stream processors,  24 ,  25  undertaking police functions. In the illustrative example of  FIG. 2 , the device  24  is fitted to the outside interface  16  in the outgoing direction, and the device  25  is fitted to another interface  20  in the incoming direction. 
   The access router is supervised by a management unit  26  which can consist of a microcomputer or a work station which executes routing software serving in particular to configure the routing table of the forwarding engine  22  and the stream processors  24 ,  25  and to exchange control or protocol information with them. These commands and exchanges are effected by way of an appropriate software programming interface (API). 
   Most of the existing packet routing and forwarding software is readily available in the Unix environment, but its performance is customarily limited on account of the frequent interruptions of the operating system. It is much faster to use a real time operating system such as VxWorks, but this complicates the implementation of the routing software. 
   The role of the stream processors  24 ,  25  is to assist the non-real time operating system (such as Unix), on the basis of which the management unit  26  functions, in the complex tasks for manipulating the streams which require real time performance (forwarding, filtering, enciphering, etc.). These processors implement a certain number of tools for manipulating the streams which may be linked dynamically according to any combination so as to perform the task required. This configuration can be achieved through the Unix operating system by calling the API functions, thereby greatly facilitating the setting up of new functionalities by the programmer. 
   As illustrated diagrammatically by  FIG. 1 , one of the tasks performed by the stream processor  24  of the outside interface  16  of the access router  15  consists in transmitting each packet to the concentrating router  12  while appending a digital signature (block  40 ) thereto. This signature attests that the packets in question have been subjected to the other stream control operations (block  39 ) performed by the processor  24 . 
   The corresponding interface  28  of the concentrating router  12  comprises a module for analyzing the packets received on the line  17  so as to make sure that the signature is present. 
   This signature technique advantageously makes it possible to decentralize the stream control operations necessary for the contractual relations between the manager of the concentrating router  12 , which provides the service of attachment to the shared network  10 , and the subscribers whose installations  13  are linked to this concentrating router  12 . In the conventional embodiments, these stream control operations are performed at the level of the concentrating router. This results in considerable complexity of the concentrating router when it is attached to a fairly large number of private installations, and a lack of flexibility for the subscribers when modifications are required. 
   By performing these stream control operations at the level of the access routers  15 , great flexibility is afforded in this regard. The signing of the packets then guarantees to the service provider that the line  17  does not send him valid packets which depart from the contractual framework with the subscriber. If such a packet were to appear, the interface  28  of the concentrating router  12  would simply eliminate it after having noted the absence of the appropriate signature. 
   Various conventional processes may be used to construct and analyze the signature of the packets, on the basis of a secret shared between the routers  12  and  15 . The signature can in particular have the form of a code word added to the content of the packet, and calculated on the basis of all or part of this content and of a secret key, the calculation being performed with the aid of a function which is extremely difficult to invert in order to recover the secret key. It is thus possible to use a technique of hashing the content of the packet, or of just a part of this content, for example an MD5 hashing (see R. Rivest, RFC 1231, “The MD5 Message Digest Algorithm”). 
   It is also possible to use an enciphering process to form the signature of the packets. The content of the packet is then enciphered with the aid of a private key, the interface  28  of the concentrating router undertaking the corresponding deciphering with the aid of a public or private key. The unenciphered packets, or those enciphered by means of a wrong key are then destroyed at the level of the interface  28 . 
   As an option, provision may be made for the interface  28  of the concentrating router to also sign the packets which it transmits on the line  17 , and for the interface  16  of the access router to verify this signature so as to make sure that the packets received are valid. 
     FIG. 3  shows the organization of a stream processor  24  or  25  of an interface of the access router  15 . 
   The stream processor receives a sequence of incoming packets  30  each comprising a header  31  in accordance with the IP protocol, and delivers a sequence of outgoing packets  32  having a header  33  after having performed certain elementary processing operations whose nature depends on the data streams concerned. 
   The incoming packets  30  are stowed away in a packets memory  35  organized as a first in-first out (FIFO) stack. Each packet is fed to the memory  35  with a processing label  36 . The processing label initially has a specified value (0 in the example represented) for the incoming packets  30 . 
   The stream processor is supervised by a unit  37  which cooperates with a table  38  making it possible to associate a particular processing module with each value of the processing label. In the simplified example represented in  FIG. 3 , the stream processor comprises an assembly of five processing modules M 1 -M 5  effecting elementary processing operations of different kind. 
   After the execution of an elementary processing operation, the supervisory unit  37  consults the packets memory  35 . If the latter is not empty, a packet is extracted therefrom according to the FIFO organization. The supervisory unit  37  consults the table  38  to determine which processing module corresponds to the label of this packet. The unit  37  then activates the module in question so that it performs the corresponding elementary processing operation. In certain cases, this elementary processing operation may entail a modification of the content of the packet, in particular its header. 
   It will be understood that the “extraction” of the packet, to which reference is made, is an extraction in the logical sense from the FIFO memory. The packet is not necessarily removed from the memory. The addresses of the packets in the memory  35  can be managed in a conventional manner by means of pointers so as to comply with the FIFO organization. The activated processing module can be furnished simply with the address of the current packet so as to perform the required reads, analyses, modifications or deletions as appropriate. 
   The first processing module M 1 , associated with the initial label  0 , is a filtering module which analyzes the address field and/or protocol definition field and/or port field of the IP header of the packets. With the help of an association table T 1 , the filtering module M 1  delivers a second processing label which identifies a string of elementary processing operations which will subsequently have to be performed on the packet. After having determined the second processing label for the packet extracted from the memory  35 , the filtering module M 1  stows away the packet in the memory  35  again, with the second processing label. The next elementary processing operation will then be executed when the packet is again extracted from the memory. 
   The module M 2  is a module for counting the packets relating to certain streams. In the case of the association table  38  represented in  FIG. 3 , this module M 2  is called for the processing labels  2  and  4 . When it processes a packet, the module M 2  increments a counter with the number of bytes of the packet, or else with the value 1 in the case of a packets counter. The counter can be made secure, in particular if it serves for the billing of the subscriber by the manager of the network  10 . In the case of a secure counter, requests are regularly made to the access provider to obtain transmission credits, the relevant packets being destroyed if the credit is used up. 
   The module M 3  of  FIG. 3  is a priorities management module. In the case of the association table  38  represented in  FIG. 3 , this module M 3  is called for the processing label  3 . The module M 3  operates on the TOS (“Type of Service”) field of the IP header of the packets. The TOS is used in the network to manage forwarding priorities so as to provide a certain quality of service on certain links. The TOS field can be changed according to prerecorded tables. These tables can be defined under the control of the access provider so as to prevent packets being inappropriately transmitted with a high priority, which might disturb the network. 
   The elementary processing operation performed last on a packet of the memory  35  is either its destruction (module M 4  activated by the label  8 ), or its resubmission to the output of the stream processor (module M 5  activated by the label  5  or  9 ). The module M 4  can be used to destroy packets having a certain destination and/or a certain origin. 
   The modules M 2  and M 3 , which do not terminate the processing operations to be undertaken in respect of a packet (except in the case of destruction), each operate with a label translation table T 2 , T 3 . This translation table designates, for the processing label extracted from the memory  35  with the current packet, another processing label designating the next elementary processing operation to be undertaken. The elementary processing operation undertaken by this module M 2  or M 3  terminates with the associating of the packet with this other processing label and the reinjecting of the packet thus processed into the memory  35 . 
   In this way, highly varied combinations of processing operations can be performed on the various data streams passing through the processor. 
     FIG. 4  shows a simplified example corresponding to the tables  38 , T 1 -T 3  represented in  FIG. 3 . The incoming packet  30 , associated with the first label  0 , is firstly subjected to the filtering effected by the module M 1 . 
   In the particular case considered, the stream processor  24  counts the packets transmitted from a source address AS 1  to a destination address AD 1  and a port P 1 , and modifies the TOS field of these packets before delivering them on the line  17 , this corresponding to the upper branch of the graph of  FIG. 4 . Moreover, the stream processor  24  counts the packets emanating from a source address AS 2  heading for a port P 2  before destroying them, this corresponding to the lower branch of  FIG. 4 . The other packets are simply delivered to the line  17 . The default value (9) of the processing label returned by the module M 1  therefore simply designates the output module M 5 . If the module M 1  detects in the packet extracted from the memory  35  the combination AS 1 , AD 1 , P 1  in the relevant address and port fields, it returns the packet with the processing label  2 . If the values AS 2 , P 2  are detected in the address and port fields, it is the label  4  which is returned with the packet. 
   These labels  2  and  4  both correspond to the counting module M 2 . The label will also designate for this module the memory address of the counter which has to be incremented. The table T 2  with which the module M 2  operates will make it possible at the end of processing to perform the return to the next module to be activated (M 3  designated by the label  3  for the packets whose TOS has to be changed, M 4  designated by the label  8  for the packets to be destroyed). 
   The module M 3  receives packets with the processing label  3 , and returns them with the label  9  after having made the required modification of the TOS field. 
   From this simplified example it can be seen that the stream processor makes it possible, through the identification of a stream by the filtering module M 1 , to perform various combinations of elementary processing operations in a relatively simple and fast manner. 
   A main advantage of this way of proceeding is the flexibility of the operations for configuring the stream processor. The tables  38 , T 1 -T 3  which define any graph of elementary processing operations, such as the one represented in  FIG. 4 , can be constructed relatively simply and with a small real time constraint by means of the management unit  36  through the API. The same holds in respect of the information enabling the modules M 1 -M 5  to perform their elementary processing operations (description of the counts to be performed by the module M 2 , way of changing the TOS fields by the module M 3 , etc.). 
   In practice, the stream processor may comprise various processing modules other than those represented by way of example in  FIGS. 3 and 4 , according to the requirements of each particular installation (for example, module for managing the output queues, address translation module, etc.) 
   The function of signing the packets transmitted, which was described earlier, can form part of the elementary processing undertaken by the output module M 5 . In a typical embodiment of the access router, the stream processor  24  will be included in an application specific integrated circuit (ASIC) organized around a microcontroller core. This embodiment allows there to be no physical access between the stream control modules  39  (at least those which pertain to the relations between the subscriber and the manager of the network  10 ) and the module M 5  which is responsible for signing the packets, corresponding to the block  40  of  FIG. 1 . This improves the security of the link from the viewpoint of the manager of the network.