Data dissemination networks differ from messaging networks in that the former are centered around data which they distribute to users whereas the latter are centered around messages which they distribute to recipients. Quite often, in a data dissemination network, each data consumer is interested in one or more types of data in particular. He therefore wishes to receive all data of this type or these types which are sent on the network. To this end, he subscribes to the types of data which are of interest to him, which is why he is called a subscriber. A subscriber does not know the producer or producers of the data that he is consuming in the network—he is also said not to know the publisher of said data. Likewise, the publisher does not know the subscribers to the types of data that he produces. Thus, there is no relationship directly established between the producers and the consumers, which present themselves independently to nodes forming the network. This is the reason that the coupling between a subscriber and a publisher in a dissemination network is said to be loose. By contrast, in a messaging network, the sender and the receiver of a message know one another. For this reason, the coupling is said to be tight.
Data dissemination networks are more particularly used in military systems. These systems are characterized firstly by the heterogeneity of the applications and of the means of communication implemented. By way of example, a wired network linking static ground platforms may be involved, said wired network itself being connected to radio networks linking mobile platforms, be they ground-based, naval or airborne. This heterogeneity limits the performance of the network in terms of passband, latency and bit rate. These systems are secondly characterized by the instability of the topology of the network. It is because, since each platform takes a node of the network onboard, the nodes on a mobile platform are able to join or leave the network at any moment according to the operational constraints on the field. Applications can also move from one dissemination mode to another, for example in the case of a change of operations. This instability affects the means of communication. The routing tables, commonly called forwarding tables and providing a local indication for a node regarding the other nodes to which it may need to transfer data which it receives, must continually be updated according to the changes in the topology of the network. These constraints concerning the heterogeneity of the means of communication and instability of the topology of the network need to be taken into account quite particularly in order to disseminate the data effectively while retaining a reasonable signaling/payload ratio.
In order to attempt to overcome the difficulties posed by the heterogeneity of interchanges, web technology based notably on the SOAP (simple object access protocol) protocol has been introduced into military systems. By way of example, it is possible to cite the WSDL (Web Service Description Language) language for describing interfaces for accessing a service or else the WS-Addressing (Web Service Addressing) standard for specifying the format for addressing messages or else the WS Security (Web Service Security) standard for safeguarding interchanges. These standards are themselves defined from other standards such as the XML (eXtensible Markup Language) language. In fact, this involves the use of web standards to define an interoperability framework between the nodes.
In order to overcome the difficulties posed by the instability of the dissemination topology, the RTPS (Real-Time Publish-Subscribe) protocol is the most promising track. RTPS is the interoperability protocol of the DDS (Data Distribution Services) standard. Proposed by the OMG (Object Management Group), DDS and RTPS are indeed one of the most accomplished standard propositions. The DDS standard proposes defining the data dissemination topology by initially defining DDS nodes. Each DDS node subsequently uses the RTPS protocol to first of all discover the other nodes and to then discover the instances of production and consumption of data declared dynamically on these nodes. To that end, messages known as “signaling” messages are interchanged between the DDS nodes. However, RTPS is based on the premise that the support network is a local area network (LAN) using the IP (Internet protocol) protocol, which implies notably that it is homogeneous and stable and that its passband is constant and almost unlimited. Furthermore, it is effectively found to be the case that RTPS is not suited to an extended heterogeneous network of WAN (Wide Area Network) type, such as a military system. RTPS defines a protocol for discovering the nodes of the dissemination network which is called SDP (Simple Discovery Protocol) and which can be deployed only on LAN-networks of small or medium size. The principle is based on the periodic sending of information to a static list of recipient nodes, with neither history nor fault tolerance. Moreover, the mechanisms for interchanging information concerning the DDS subscriptions between the nodes are based on the sending of messages to this same list of nodes upon an event, an event being addition, deletion or modification of a subscription. This can lead to an explosion in the number of signaling messages interchanged, for example in the case of an unstable dissemination network in which numerous applications enter and leave the network. This is why it is still said that RTPS “does not support the scaling” of heterogeneous WANs or else that it is not “scalable”.
More generally in the rest of the present document, the terms “network” and “dissemination network” will be used indiscriminately. The dissemination network represents all of the dissemination nodes used and the means of communication which they use to communicate.