Abstract:
Computer networks with multiple nodes are often required to deliver packets to all nodes in the network, commonly referred to as “flooding.” Flooding is used to deliver multicast and broadcast packets generated by application, network and other layers of the networking stack. Flooding can be done very reliably but less efficiently via node to node “unicast” transmissions, or very efficiently but less reliably via “broadcast” transmissions. In order to balance reliability with efficiency, this invention defines a threshold for the number of neighboring nodes as seen by a given node prior to a flooding operation to determine whether data should be unicast or broadcast. Below that threshold, unicast is used; at or above that threshold, broadcast is used. The invention also incorporates knowledge of nodes seen in turn by neighbor nodes as part of this decision.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a non-provisional of U.S. Provisional Application Ser. No. 61/760,887, filed Feb. 5, 2013, whose disclosure is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention concerns tactical and other highly mobile communications networks. Such networks are distinguished by their ability to self-organize and heal connections as radio nodes enter and leave each others&#39; direct communications ranges, with minimal impact to the performance of other nodes on the network. 
       DESCRIPTION OF RELATED ART 
       [0003]    Flooding is a common network operation, which occurs when a single packet is sent to every other node in the network. Each node that receives that packet, and hasn&#39;t seen it previously, forwards it to every other known neighbor node except for the originator. 
         [0004]    The forwarding of flood packets can be performed in two distinct ways—using either unicast or broadcast transmissions. Unicast is point to point, one node to another, with an acknowledgment packet sent in return. Unicast is a very reliable means of communicating, but it can be problematic in very dense networks because the number of unicast packets needed to perform a flooding operation increases quadratically with the number of nodes comprising the network. Broadcast is point to multipoint, one node to every other node within range. In a broadcast, the number of packets necessary to flood increases linearly with the number of nodes, so the flood is more efficient. However, broadcasts do not acknowledge receipt of packets, so they are considered less reliable. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of the invention to manage messaging to balance reliability with network congestion. 
         [0006]    To achieve the above and other objects, the present invention is directed to an improved method of handling messages broadcast across a digital network. In each such network, a series of microcomputer-based networking devices communicate with one another. Each of these network nodes will receive messages from other nodes and will transmit or relay messages to additional nodes. 
         [0007]    In the preferred embodiment of the invention, the network is a dynamic mesh network, which has to effectively manage both very dense and very sparse network configurations. The density of the network at any given node is defined by the number of nodes that can be directly accessed by that node, i.e., that node&#39;s neighbors. In an active mesh network, nodes can be constantly moving or influenced by environmental and topographic factors. This can break direct links from a node to its neighbors, and introduce new direct linked neighbors, as nodes are repositioned. 
         [0008]    The number of nodes directly linked to a given node is the “node density” for that given node. The Flooding Threshold protocol establishes a flooding algorithm that defines a threshold of node density. In the preferred embodiment, the network mesh maintains an active list of neighbor nodes for each node. The node threshold is compared to this neighbor count to determine the local density. When the count of neighbors falls below that local density, unicast packets are sent, one to each neighbor, to achieve the flood/forward. At or above that local density, broadcast is used—a single packet sent to every neighbor. 
         [0009]    The Flooding Threshold protocol is further refined by considering the node density as seen by each neighbor node. Rather than broadcasting when the local node count is above the node threshold, each neighbor&#39;s reported node density is also compared to the node threshold. When a neighbor&#39;s density is below the threshold, unicast is used for the flood to that node. Only when both the local density and a neighbor&#39;s density are equal to or above the node threshold is a broadcast used. This broadcast is sent only once for each flood packet. 
         [0010]    Some implementations of the invention also provide that all nodes maintain a count of neighbors and report any changes in count to their neighbors without sending excessive notifications of neighbor count changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A preferred embodiment of the invention will be set forth in detail with reference to the drawings, in which: 
           [0012]      FIG. 1  illustrates Unicast Flooding; 
           [0013]      FIG. 2  illustrates Broadcast Flooding; 
           [0014]      FIG. 3  illustrates the Flooding Threshold Flow Chart; and 
           [0015]      FIG. 4  illustrates the Peer Count Notification Flow Chart. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which like reference numerals refer to like elements or steps throughout. 
         [0017]    Many messages are sent via unicast data packets. For a unicast packet, a message is constructed by one node and sent directly to the network address of the receiving node. This packet is, in turn, acknowledged by the receiving node, via an acknowledgment packet sent back to the originating node. If the originating node does not receive the acknowledgment within a defined window of time, resends the packet. Thus, this is a very reliable mechanism, as the network knows when data is not received. 
         [0018]    However, some kinds of messages are sent to every node in a network, one packet that needs to be delivered to every node in the network, commonly referred to as “flooding.” This is performed by forwarding a packet from any node to every other node that it is directly linked with (i.e., neighbor node), except to the node from which the packet was sent. The links between the nodes can be based on a variety of wired or wireless media such as Ethernet, fiber-optic, IEEE 802.11, 802.15.4, 802.16, cellular, etc. The process of forwarding continues until as many nodes as possible, and ideally all nodes comprising the network, have received a copy of the packet. In the preferred embodiment of the invention, the network is a wireless mesh network. However, the invention is applicable to other networks, particularly those with dynamically changing topologies or dense clusters of network nodes. 
         [0019]    The forwarding of flood packets can be performed in two distinct ways—using either unicast or broadcast methods. In a unicast transmission the packet is sent to one specific node that is identified by a unique address. Unicast transmissions usually use an Automatic Repeat reQuest (ARQ) mechanism. As part of this mechanism the transmission is acknowledged, meaning the receiving node replies with a special frame, known as the Acknowledgment frame, that confirms the receipt of the transmission. In addition, successive transmission attempts (Retry frames) are made if no acknowledgment is received as the transmitting node tries to reliably deliver the packet. In unicast flooding, the forwarding node sends a unicast packet to all of its neighbors, except to the node that sent the packet. 
         [0020]    This is illustrated in  FIG. 1 , Unicast Flooding  100 . In this example, node A is going to deliver a packet to each of its neighbors. This happens serially; a packet is constructed and sent to node B  102 , then to node C  104 , then to node D  106 , node E  108 , and node F  110 . In each case, node A will receive and process an Acknowledgment frame from each of the neighbor nodes. If one of these packets does not arrive on time, it will retry sending the packet to that neighbor again. This is inefficient as it requires the generation and transmission of replicas of the packet for each neighbor, a minimum of five packets sent and five&#39;packets received by node A. The number of replicated packets, and therefore the use of bandwidth and other related resources increases exponentially with the density of the network, where network density is defined as the average number of neighbors per node. 
         [0021]    In a broadcast transmission, the packet is sent to all nodes that listen to a common physical medium and therefore can simultaneously receive the packet. A broadcast packet uses a special broadcast destination address, which all operating nodes following a given protocol in the network will recognize. A broadcast transmission is one time only, without the need of packet replication or acknowledgment, and is therefore more efficient in use of bandwidth and other related resources. On the other hand, due to the lack of an ARQ mechanism utilizing Acknowledgments and Retries, a broadcast transmission is not known by the sender to be received, and is inherently less reliable. There is no guarantee that a broadcast packet reaches each and every neighbor of the forwarding node. As a result, efficiency occurs at the expense of reliability as no ARQ mechanism is employed. 
         [0022]    This is illustrated in  FIG. 2 , Broadcast Flooding  200 . As in the previous case, node A wants to send a packet to every node it can reach. Using the broadcast flooding mechanism, it sends a single packet  202 . Nodes B, C, D, E, and F should receive this packet just as before, but in the space of a single transmission, rather than ten. However, if there is some interference or other problem during the transmission, node A will not know that one or more of the neighbor nodes might have missed the packet. Reliability is diminished in exchange for efficiency. 
         [0023]    The situation actually gets more complex in the preferred embodiment, which is running on a mesh network. During a flooding operation, each node receiving a flooded packet will forward that packet, if it has not been seen yet by the receiving node, to every node it can see other than the originator. So for the case of  FIG. 1 , node A will send each packet  5  times. Each of the other nodes will, in turn, send one acknowledgment packet back to node A, and then proceed to relay the packet to any of the other nodes they can see. Those nodes will not relay the packet if they have received it, but they still have to acknowledge it. If every node can see every other node, this would be 40 packets total, or in general, N*(N−1)*2 packets, quadratic growth as the network density increases. 
         [0024]    In the broadcast flooding case, far fewer packets are required. The original node will broadcast the packet once, and each of its N−1 neighbors will forward it once, leading to a total of N packets sent, which is only linear growth. This meshing behavior lends a natural redundancy to the broadcast process. There is still no guaranteed delivery of the packet, but, for example, if node F somehow missed the initial packet from node A, it is likely to be able to get the relay either from node D or from node B. And this redundancy increases as the network density increases, which is exactly when broadcast over unicast is most efficient. 
         [0025]    The Flooding Threshold is the number of peers that indicates the density required to use the broadcast flooding. To better assess the real network density, both sides of the link between peers must have at least the threshold number of peers to avoid using unicast flooding to that peer. 
         [0026]    Nodes periodically report the number of peers they have to their peers and always report their peer count when they cross the threshold in either direction. Normally, the periodic reporting is only done when a link is idle during the reporting interval. When a node adds or removes peers, it must check if it has crossed the threshold and then it must immediately notify all of its peers of the new count regardless of current traffic on the link between the peers. This notification uses a unicast message to each peer. If the attempt to report the current peer count fails, the node will note this during the periodic peer reporting and send the peer report even if the link was in use during the reporting interval. 
         [0027]      FIG. 3  illustrates the decision process  300 . Start with the Flooding Threshold set to N  302 . Is the operation a flooding operation  304 ? If not, send the packet via unicast  306 , and then wait for the next packet  302 . When flooding is required, consider each neighboring node independently  308 . Are there more neighbors to process  310 ? If so, does the current node have N or more neighbors  312 ? If not, simply unicast to each neighbor  314 . If the current node does have more than N neighbors, the processor will check the stored neighbor&#39;s neighbor count. If that is less than N  316 , a unicast is still used for that neighbor  314 . Otherwise, if we have not yet broadcast  318 , the current node does a single broadcast to neighbors  320 . The loop still continues  308 , unicasting to any neighbor with less than N of its own neighbors. 
         [0028]    The final part of the process is neighbor count management. As nodes move around in a dynamic network, they can gain and lose neighbor nodes on a regular basis. Each node maintains a count of neighbor nodes, and when that count changes, it will send that new count of neighbors to each of its neighbor nodes.  FIG. 4  illustrates the peer count notification process. Three events trigger a possible response. A peer may be (1) added or (2) removed  404  or (3) it may be time for a periodic peer count report  424 . When a peer has been added or removed, the node checks its internal record of its neighbors  408  to determine if the last reported number of peers was below the threshold  414  and that the current count of peers is above the threshold  414 . If both are true, it sends a unicast message to the neighbor to update the peer count  416 . If it last reported greater than or equal to the threshold of peers, it then checks if it has less peers than the threshold  418 . If this is true, it also sends a unicast message the neighbor to update the peer count  416 . If not, it then checks if it has transmitted any messages to this peer in the last periodic interval  426 . If there is no transmission, it sends a unicast message to update the peer count and test the link  416 . If it has transmitted messages to this peer in the last periodic interval, it continues to check other peers if the process was triggered by adding or removing a peer  404 . If the process was triggered by the periodic interval occurring  424 , a unicast message is sent. 
         [0029]    While a preferred embodiment of the present invention has been set forth above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the invention. For example, recitations of specific technologies are illustrative rather than limiting. Therefore, the present invention should be construed as limited only by the appended claims.