Abstract:
A method of exchanging information between a first network implementing a routing protocol using static routing tables and a second network implementing a routing protocol using dynamic routing tables and including stages of exchanging identification messages and of exchanging routing table update messages, the method comprising the steps of: a first routing element associated with the first network issuing to a second routing element associated with the second network identification messages containing identifiers of network elements to which the first routing element knows it is connected either directly or indirectly, and in the second routing element updating the routing table from the identification messages coming from the first routing element.

Description:
BACKGROUND OF THE INVENTION 
     Networks of a first type are known in which information is routed between different elements of the network by using static routing tables. For each element of the network, a routing table lists the portions of the network that are directly or indirectly connected to the network element in question, together with the interfaces of said network element giving access to these various portions of the network. A routing table is said to be static when any change to the network assumes that a corresponding change will be made by an operator to the routing tables of the network elements. 
     Networks of a second type are known in which information is routed between the various elements of the network by using dynamic routing tables. One such routing protocol is the link state hierarchical routing protocol, known as open shortest path first (OSPF), which constitutes specifically the subject matter of the Internet Engineering Task Force (IETF) technical report RFC 2328. This type of routing protocol provides for a state in which identification messages and routing table update messages are interchanged. Each identification message includes a neighborhood identification field normally dedicated to receiving the identifiers of neighboring network elements that are connected directly to the network element issuing the identification message, and serving to inform the neighboring network elements that the emitting network element is active. The update messages contain, in particular, in usual manner, the identifiers of network elements to which the message-emitting network element knows it is connected either directly or indirectly. 
     There are proposals for installing on-board networks in certain airplanes, comprising an avionics network of the first type that is secure and that is used to operate the airplane, and a public network of the second type that is less secure and that is used by the crew for obtaining information about the flight or the passengers, about the airports used, . . . , and also to enable passengers to have access to entertainment means. The public network is also used by maintenance teams on the ground for obtaining flight parameters after the airplane has landed. The avionics network is connected to the public network via at least two first routing elements selectively associated with the avionics network, and at least one second routing element associated with the public network and connected to the first routing elements. 
     Ideally, in order to ensure full inter-operability between the two networks, it would be necessary for the first routing element to be capable of implementing the routing protocol on the basis of dynamic tables and of exchanging identification messages and update messages with the second routing element. Nevertheless, such exchanges of information would burden the first routing devices in quite significant manner, particularly since as a general rule such routing devices have computing resources that are relatively limited. 
     In addition, in order to ensure that the network is reliable, provision is made for the two first routing elements to be connected to the first network in alternation. It is thus essential for the second routing elements to be able to identify which first routing element is active in order to perform effective routing. 
     OBJECT OF THE INVENTION 
     An object of the invention is to propose a method of exchanging information between a network using a routing protocol based on static routing tables and a network using a routing protocol based on dynamic routing tables, which method requires only a limited amount of information transfer in order to update the dynamic routing tables while nevertheless enabling information to be routed reliably. 
     BRIEF SUMMARY OF THE INVENTION 
     To this end, the invention provides a method of exchanging information between a first network and a second network, each made up of network elements, and connected to each other by at least two first routing elements that are selectively associated with the first network, and by at least one second routing element that is associated with the second network and that is connected to the first routing elements. The first network implements a routing protocol using static routing tables and the second network implements a routing protocol using dynamic routing tables. The method comprises the steps of:
         each first routing element issuing to the second routing element identification messages each having a neighborhood identification field containing the identifiers of network elements to which the first routing element knows it is connected directly or indirectly;   in each first routing element, processing the identification messages coming from the second routing element solely as indicating that the second routing element is active; and   in the second routing element, updating the routing table from the identification messages coming from the first routing element.       

     In this case, the identifiers of network elements to which the first routing element knows it is connected the network addresses of said network elements, i.e. the addresses of the portions of the network (or networks) to which said network elements belong. 
     Thus, the identification messages issued by the first routing element comprise both conventional identification message information and information enabling the routing table of the second routing element to be updated. Because this information, which ought normally to appear in the update messages, is incorporated in the identification messages, there is no longer any need for the update messages. The work load on the first routing element is then relatively small and the volume of information transiting between the first routing elements and the second routing elements is small. The neighborhood identification fields of the identification messages issued by the first routing elements also enable the second routing element to determine which of the two first routing elements is actually connected to the first network. 
     Advantageously, the second routing element is configured not to issue update messages to the first routing element. 
     Since update messages are neither issued nor used by the first routing elements that possess respective static routing tables, and possibly default routes for information having the second network as its destination, there is no need for the second routing element to send update messages to the first routing element, thereby making it possible to further reduce the work load on the first routing element and the volume of information exchanged over the link between the first routing element and the second routing element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention appear on reading the following description of a particular, non-limiting implementation of the invention. 
       Reference is made to the sole accompanying FIGURE which is a diagram of a network implementing the method in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the FIGURE, the method of the invention is described below as applied to a network given overall reference  1  and located on board an aircraft, for example. 
     The network  1  comprises a network  2  having network elements  3  such as servers or other computer devices, and a network  4  comprising network elements  5  such as servers or other computer devices. In this case, the network  2  is a public network used by the crew to obtain information concerning the flight, the passengers, the airports visited, . . . , and also to give passengers access to entertainment means. In this case the network  4  is used to operate the airplane itself. The network  4  is a highly secure network, while the network  2  is a less secure network. 
     Two protection and routing elements  6   a  and  6   b  are connected to the network  1  via a selector device  7 . 
     Each protection and routing device  6  is itself known and comprises a router  8  with an internal interface  9  connected to an internal interface  10  of a demilitarized zone (DMZ) type filter element  11 . The router  8  also has an external interface  12  connected to the selector device  7  and an internal interface  13  suitable for use in maintenance operations. The filter element  11  possesses external interfaces  14 ,  15  and an internal interface  16  suitable for use in maintenance operations. 
     The external interfaces  14  of the filter elements  11  of the protection and routing devices  6   a  and  6   b  are connected to the interfaces  17  of the routing devices  19   a  and  19   b  respectively. The external interfaces  15  of the filter elements  11  of the protection and routing devices  6   a  and  6   b  are connected to the interfaces  18  of the routing devices  19   a  and  19   b  respectively. The links between firstly the protection and routing devices  6   a  and  6   b  and secondly the routing devices  19   a  and  19   b  respectively are privileged in the routing tables. 
     The routing devices  19   a  and  19   b  are connected to the network  2 . 
     The selector device  7  includes means for switching the link from the network  4  to one or the other of the protection and routing devices  6   a ,  6   b  (with the protection and routing device  6   a  being shown connected to the network  4  in the FIGURE). 
     The network  4  operates using a routing protocol based on static routing tables. The protection and routing devices  6   a  and  6   b , and likewise the network elements  5  that perform routing functions, thus have memories containing the routing tables that are associated with them respectively. In this case, each routing table has defined routes for reaching network elements  4  and a default route for reaching the network elements  2 . 
     The network  2  operates using a routing protocol based on so-called dynamic routing tables. Each routing table contains routes which associate an outlet interface with a network to be reached (or a network portion or a sub-network) in the form of a network address associated with a network mask. In this case, the routing protocol is the OSPF protocol which provides for the network elements  2  that perform a routing function (including the routing devices  19   a  and  19   b ) to exchange:
         identification messages each having a neighborhood identification field normally dedicated to receiving the identifiers (specifically the IP addresses) of neighboring network elements directly connected to the network element issuing the identification message and informing said elements that the issuing network element is active (these messages are known as “HELLO” packets in the OSPF protocol); and   routing table update messages, which messages normally contain the identifiers of the network elements to which the message-issuing network element knows it is connected either directly or indirectly (these messages are known as link state messages LSA, LSU, or LSR in the OSPF protocol). These identifiers are the network addresses of said network elements, which network addresses are associated with network ranges.       

     The protection and routing devices  6  are configured:
         to issue to the routing devices  19  identification messages having respective neighborhood identification fields containing the identifiers of the network elements to which the protection and routing device knows it is connected either directly or indirectly, i.e. the network addresses: of the interfaces  9  (where the interfaces  9  have the same address as the associated interfaces  10 ; it should be observed that two interfaces that are connected together have the same network address and that this address is mentioned once only); of the interfaces  12  (if the protection and routing device  6  in question is connected to the selection device  7 ); of the interfaces  13 ,  14 , or  15  (depending on whether the protection and routing device  6  in question is connected to the routing device  19   a  or  19   b ; the network addresses of the interfaces  14  and  15  being the same as the addresses of the interfaces  17  and  18 , respectively); and of the interface  16 , and the network address of the network  4 ;   to process the identification messages received from the routing devices  19  merely as being indicative that the routing devices  19  are active.       

     Thus, the neighborhood identification field issued by the protection and routing device  6   a  contains:
         the addresses of the networks: to which the interfaces  9  and  12  belong (since the protection routing device  6   a  is connected to the selector device  7 ); to which the interfaces  13  and  14  belong (the protection and routing device  6   a  having a priority connection to the interface  17  of the routing device  19   a ; if the protection and routing device  6   a  receives identification messages only from the routing device  19   b , then the network address of the interface  15  of that device would be given); and to which the interface  16  belongs; and   the network address of the network  4 .       

     The neighborhood identification field issued by the protection and routing device  6   b  contains the network addresses of the interface  16  and of the interfaces  9 ,  13 , and  14  (the protection and routing device  16  being connected on a priority basis to the interface  17  of the routing device  19   b ). 
     Since the protection and routing device  6   b  is not connected by the selector device  7  to the network  4 , its interface  12  is not active, so the network address of the network  4  and the network address of the interface  12  do not appear in the neighborhood identification field. Thus, the routing device  19   a  is informed as to which protection and routing device  6  is connected to the network  4 . 
     The routing devices  19  are configured:
         to update their routing tables from the identification messages received from the protection routing devices  6 ;   not to issue routing table update messages to the protection and routing devices  6 ; and   to exchange identification messages and update messages in conventional manner with the other network elements  3  that perform a routing function.       

     Each routing device  19  needs to contain in its memory a static correspondence table matching the network addresses that appear in the neighborhood identification fields of the identification messages from the protection and routing devices  6  with the network ranges of said addresses, which information cannot be conveyed in the neighborhood identification field of an identification message. To ensure that the OSPF protocol operates properly, it is also necessary for each route to be stored in association with a weight in this static correspondence table. Since only one route is possible for reaching the network  4 , the weight is determined arbitrarily, and in this case is equal to 1. 
     It should be observed that the network  4  is globally identified in the neighborhood identification fields of the identification messages issued by the protection and routing devices  6 . This makes it possible to avoid the protection and routing devices  6  transmitting information about the topography of the network  4  over the network  2 . 
     The composition of the network  4  is thus kept secret from the network  2 . This improves the security of the network  4 . 
     Naturally, the invention is not limited to the implementation described and variations can be applied thereto without going beyond the ambit of the invention as defined by the claims. 
     In particular, the configuration of the network could be different from that shown, and it could include other quantities of protection and routing devices  6  and/or of routing devices  19 . 
     The selector device may be an electrical switch connected to the links between the network  4  and the protection and routing devices  6 , or else selection can be the result of allocating an address to only one of the protection and routing devices  6 . 
     The protection and routing devices  6  can be connected to the network  4  via a proxy server which in addition to verifying the content of data streams, also serves to perform protocol conversion in the event of the two networks not using the same communications protocols.