Patent Publication Number: US-2013250811-A1

Title: Dynamic division of routing domains in reactive routing networks

Description:
RELATED APPLICATION 
     The present application claims priority to U.S. Provisional Patent Application No. 61/614,703, filed Mar. 23, 2012, entitled TECHNIQUES FOR USE IN REACTIVE ROUTING NETWORKS, by Vasseur, et al., the contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to communication networks, and, more particularly, to reactive routing in communication networks. 
     BACKGROUND 
     Low power and Lossy Networks (LLNs), e.g., sensor networks, have a myriad of applications, such as Smart Grid (smart metering), home and building automation, smart cities, etc. Various challenges are presented with LLNs, such as lossy links, low bandwidth, battery operation, low memory and/or processing capability, etc. Routing in LLNs is undoubtedly one of the most critical challenges and a core component of the overall networking solution. Two fundamentally and radically different approaches, each with certain advantages and drawbacks, have been envisioned for routing in LLN/ad-hoc networks known as: 
     1) Proactive routing: routing topologies are pre-computed by the control plane (e.g., IS-IS, OSPF, RIP, and RPL are proactive routing protocols); and 
     2) Reactive routing: routes are computed on-the-fly and on-demand by a node that sends one or more discovery probes throughout the network (e.g., AODV, DYMO, and LOAD are reactive routing protocols). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identically or functionally similar elements, of which: 
         FIG. 1  illustrates an example communication network; 
         FIG. 2  illustrates an example network device/node; 
         FIGS. 3A-3J  illustrate examples of dynamic division of a reactive routing network into sub-domains as described herein; 
         FIG. 4  illustrates an example simplified procedure for dynamic division of a reactive routing network into sub-domains, particularly from the perspective of a transit node; and 
         FIG. 5  illustrates another example simplified procedure for dynamic division of a reactive routing network into sub-domains, particularly from the perspective of a requesting node. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     According to one or more embodiments of the disclosure, a reactive routing network may be dynamically divided into reactive routing network sub-domains that comprise a plurality of nodes having bounded route request (RREQ) scopes (e.g., search-domains) that are limited to a particular path length. A transit node may receive a RREQ from an originating node within the first reactive routing network sub-domain for a target node determined by the originating node to be beyond the bounded RREQ scope of the originating node. The transit node may then discover a route from the transit node to the target node, and return the route to the originating node. In this manner, the transit node may establish a complete route between the originating node and the target node. 
     According to one or more additional embodiments of the disclosure, a node within a reactive routing network may receive a segmentation message from a capable node (e.g., a transit node, a LBR, etc.), and in response, establish a bounded route request (RREQ) scope for any RREQ originated by the node which is limited to a particular path length. As such, the node may forward RREQs to a transit node for any target node not identified by the node as being within the bounded RREQ scope of the node. 
     Description 
     A computer network is a geographically distributed collection of nodes interconnected by communication links and segments for transporting data between end nodes, such as personal computers and workstations, or other devices, such as sensors, etc. Many types of networks are available, ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect the nodes over dedicated private communications links located in the same general physical location, such as a building or campus. WANs, on the other hand, typically connect geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines, optical lightpaths, synchronous optical networks (SONET), synchronous digital hierarchy (SDH) links, or Powerline Communications (PLC) such as IEEE 61334, IEEE 21901.2, and others. In addition, a Mobile Ad-Hoc Network (MANET) is a kind of wireless ad-hoc network, which is generally considered a self-configuring network of mobile routes (and associated hosts) connected by wireless links, the union of which forms an arbitrary topology. 
     Smart object networks, such as sensor networks, in particular, are a specific type of network having spatially distributed autonomous devices such as sensors, actuators, etc., that cooperatively monitor physical or environmental conditions at different locations, such as, e.g., energy/power consumption, resource consumption (e.g., water/gas/etc. for advanced metering infrastructure or “AMI” applications) temperature, pressure, vibration, sound, radiation, motion, pollutants, etc. Other types of smart objects include actuators, which may be, e.g., responsible for turning on/off an engine or perform any other actions. Sensor networks, a type of smart object network, are typically shared-media networks, such as wireless or PLC networks. That is, in addition to one or more sensors, each sensor device (node) in a sensor network may generally be equipped with a radio transceiver or other communication port such as PLC, a microcontroller, and an energy source, such as a battery. Often, smart object networks are considered field area networks (FANs), neighborhood area networks (NANs), etc. Generally, size and cost constraints on smart object nodes (e.g., sensors) result in corresponding constraints on resources such as energy, memory, computational speed and bandwidth. Correspondingly, a reactive routing protocol may, though need not, be used in place of a proactive routing protocol for smart object networks. 
       FIG. 1  is a schematic block diagram of an example computer network  100  illustratively comprising nodes/devices  200  (e.g., labeled as shown, “root,” “ 11 ,” “ 12 ,” . . . “ 43 ,” and described in  FIG. 2  below) interconnected by various methods of communication. For instance, the links  105  may be wired links or shared media (e.g., wireless links, PLC links, etc.) where certain nodes  200 , such as, e.g., routers, sensors, computers, etc., may be in communication with other nodes  200 , e.g., based on distance, signal strength, current operational status, location, etc. Those skilled in the art will understand that any number of nodes, devices, links, etc. may be used in the computer network, and that the view shown herein is for simplicity. Also, those skilled in the art will further understand that while the network is shown in a certain orientation, particularly with a “root” node, the network  100  is merely an example illustration that is not meant to limit the disclosure. 
     Data packets  140  (e.g., traffic and/or messages sent between the devices/nodes) may be exchanged among the nodes/devices of the computer network  100  using predefined network communication protocols such as certain known wired protocols, wireless protocols (e.g., IEEE Std. 802.15.4, WiFi, Bluetooth®, etc.), PLC protocols, or other shared-media protocols where appropriate. In this context, a protocol consists of a set of rules defining how the nodes interact with each other. 
       FIG. 2  is a schematic block diagram of an example node/device  200  that may be used with one or more embodiments described herein, e.g., as any of the nodes shown in  FIG. 1  above. The device may comprise one or more network interfaces  210  (e.g., wired, wireless, PLC, etc.), at least one processor  220 , and a memory  240  interconnected by a system bus  250 , as well as a power supply  260  (e.g., battery, plug-in, etc.). 
     The network interface(s)  210  contain the mechanical, electrical, and signaling circuitry for communicating data over links  105  coupled to the network  100 . The network interfaces may be configured to transmit and/or receive data using a variety of different communication protocols. Note, further, that the nodes may have two different types of network connections  210 , e.g., wireless and wired/physical connections, and that the view herein is merely for illustration. Also, while the network interface  210  is shown separately from power supply  260 , for PLC the network interface  210  may communicate through the power supply  260 , or may be an integral component of the power supply. In some specific configurations the PLC signal may be coupled to the power line feeding into the power supply. 
     The memory  240  comprises a plurality of storage locations that are addressable by the processor  220  and the network interfaces  210  for storing software programs and data structures associated with the embodiments described herein. Note that certain devices may have limited memory or no memory (e.g., no memory for storage other than for programs/processes operating on the device and associated caches). The processor  220  may comprise hardware elements or hardware logic adapted to execute the software programs and manipulate the data structures  245 . An operating system  242 , portions of which are typically resident in memory  240  and executed by the processor, functionally organizes the device by, inter alia, invoking operations in support of software processes and/or services executing on the device. These software processes and/or services may comprise an illustrative routing process  244 , as described herein. Note that while the routing process  244  is shown in centralized memory  240 , alternative embodiments provide for the process to be specifically operated within the network interfaces  210 . 
     It will be apparent to those skilled in the art that other processor and memory types, including various computer-readable media, may be used to store and execute program instructions pertaining to the techniques described herein. Also, while the description illustrates various processes, it is expressly contemplated that various processes may be embodied as modules configured to operate in accordance with the techniques herein (e.g., according to the functionality of a similar process). Further, while the processes have been shown separately, those skilled in the art will appreciate that processes may be routines or modules within other processes. 
     Routing process (services)  244  contains computer executable instructions executed by the processor  220  to perform functions provided by one or more routing protocols, such as proactive or reactive routing protocols as will be understood by those skilled in the art. These functions may, on capable devices, be configured to manage a routing/forwarding table (a data structure  245 ) containing, e.g., data used to make routing/forwarding decisions. In particular, in proactive routing, connectivity is discovered and known prior to computing routes to any destination in the network, e.g., link state routing such as Open Shortest Path First (OSPF), or Intermediate-System-to-Intermediate-System (ISIS), or Optimized Link State Routing (OLSR). Reactive routing, on the other hand, discovers neighbors (i.e., does not have an a priori knowledge of network topology), and in response to a needed route to a destination, sends a route request into the network to determine which neighboring node may be used to reach the desired destination. Example reactive routing protocols may comprise Ad-hoc On-demand Distance Vector (AODV), Dynamic Source Routing (DSR), DYnamic MANET On-demand Routing (DYMO), LLN On-demand Ad hoc Distance-vector (LOAD), etc. Notably, on devices not capable or configured to store routing entries, routing process  244  may consist solely of providing mechanisms necessary for source routing techniques. That is, for source routing, other devices in the network can tell the less capable devices exactly where to send the packets, and the less capable devices simply forward the packets as directed. 
     Notably, mesh networks have become increasingly popular and practical in recent years. In particular, shared-media mesh networks, such as wireless or PLC networks, etc., are often on what is referred to as Low-Power and Lossy Networks (LLNs), which are a class of networks in which both the routers and their interconnect are constrained: LLN routers typically operate with constraints, e.g., processing power, memory, and/or energy (battery), and their interconnects are characterized by, illustratively, high loss rates, low data rates, and/or instability. LLNs are comprised of anything from a few dozen and up to thousands or even millions of LLN routers, and support point-to-point traffic (between devices inside the LLN), point-to-multipoint traffic (from a central control point such at the root node to a subset of devices inside the LLN) and multipoint-to-point traffic (from devices inside the LLN towards a central control point). 
     An example implementation of LLNs is an “Internet of Things” network. Loosely, the term “Internet of Things” or “IoT” may be used by those in the art to refer to uniquely identifiable objects (things) and their virtual representations in a network-based architecture. In particular, the next frontier in the evolution of the Internet is the ability to connect more than just computers and communications devices, but rather the ability to connect “objects” in general, such as lights, appliances, vehicles, HVAC (heating, ventilating, and air-conditioning), windows and window shades and blinds, doors, locks, etc. The “Internet of Things” thus generally refers to the interconnection of objects (e.g., smart objects), such as sensors and actuators, over a computer network (e.g., IP), which may be the Public Internet or a private network. Such devices have been used in the industry for decades, usually in the form of non-IP or proprietary protocols that are connected to IP networks by way of protocol translation gateways. With the emergence of a myriad of applications, such as the smart grid, smart cities, and building and industrial automation, and cars (e.g., that can interconnect millions of objects for sensing things like power quality, tire pressure, and temperature and that can actuate engines and lights), it has been of the utmost importance to extend the IP protocol suite for these networks. 
     As noted above, routing in LLNs is undoubtedly one of the most critical challenges and a core component of the overall networking solution. Two fundamentally and radically different approaches have been envisioned for routing in LLN/ad-hoc networks known as proactive routing (routing topologies are pre-computed by the control plane) and reactive routing (routes are computed on-the-fly and on-demand by a node that sends a discovery probes throughout the network). 
     An example proactive routing protocol specified in an Internet Engineering Task Force (IETF) Proposed Standard, Request for Comment (RFC) 6550, entitled “RPL: IPv6 Routing Protocol for Low Power and Lossy Networks” by Winter, et al. (March 2012), provides a mechanism that supports multipoint-to-point (MP2P) traffic from devices inside the LLN towards a central control point (e.g., LLN Border Routers (LBRs) or “root nodes/devices” generally), as well as point-to-multipoint (P2MP) traffic from the central control point to the devices inside the LLN (and also point-to-point, or “P2P” traffic). RPL may generally be described as a distance vector routing protocol that builds a Directed Acyclic Graph (DAG) or Destination Oriented Directed Acyclic Graphs (DODAGs) for use in routing traffic/packets  140  from a root using mechanisms that support both local and global repair, in addition to defining a set of features to bound the control traffic, support repair, etc. One or more RPL instances may be built using a combination of metrics and constraints. 
     An example reactive routing protocol is specified in an IETF Internet Draft, entitled “LLN On-demand Ad hoc Distance-vector Routing Protocol-Next Generation (LOADng)” &lt;draft-clausen-lln-loadng-05&gt; by Clausen, et al. (Jul. 14, 2012 version), which provides a reactive routing protocol for LLNs, e.g., as derived from AODV. Other reactive routing protocol efforts include the G3-PLC specification approved by the ITU, and also one described in an informative annex of IEEE P1901.2. 
     One stated benefit of reactive routing protocols is that their state and communication overhead scales with the number of active sources and destinations in the network. Such protocols only initiate control traffic and establish state when a route to a destination is unknown. In contrast, proactive routing protocols build and maintain routes to all destinations before data packets arrive and incur state and communication overhead that scales with the number of nodes, rather than the number of active sources and destinations. Some believe that reactive routing protocols are well-suited for certain Smart Grid Automated Meter Reading (AMR) applications where a Collection Engine reads each meter one-by-one in round-robin fashion. In such simplistic applications, only one source-destination pair is required at any point in time. 
     Reactive routing protocols, however, have a number of technical issues that are particularly exhibited in large-scale LLNs, such as large utility networks. It is thus important to have a robust solution for reactive routing. Therefore, various techniques are hereinafter shown and described for use with reactive routing networks to address such shortcomings. 
     Dynamic Division of Reactive Routing Networks into Sub-Domains 
     Reactive routing protocols rely on flooding the whole network with probes/messages (e.g., RREQs) to discover routes between a source and a destination within the network. Unfortunately, such network floods generate significant volumes of network traffic. Several mitigation techniques have been developed to reduce the negative effects of flooding by reducing/limiting the number of broadcast packets generated by such floods. Illustratively, these techniques may operate by attempting to limit the flood scope, the number of duplicated messages (e.g., multicast trickle), etc. Nevertheless, such network floods are still generally required for any reactive routing protocol in order to make sure that at least N probes/messages reach the destination/target. It is important to note that while N may be small in “classic” networks that have high delivery rates, N is likely to be higher in LLNs in which the Packet Delivery Ratio (PDR) is typically low. Unfortunately, these mitigation techniques lead to a trade-off between storing network state and increasing network load due to flooded messages (e.g., a RREQ broadcast). For example, storing more network state information makes it possible to reduce the number of times that the discovery process (e.g., a network flood) must be triggered, and therefore decreases the control plane overhead; however, storing more state information requires more memory to store the routing entries for each originator, especially in cases where the routes are not limited to the best next hops, but rather include full end-to-end paths from the source to the destination, which increases cost. 
     The techniques herein provide dynamic division of a reactive routing network into sub-domains by allowing a routing sub-domain to be dynamically divided into search-domains based on the observed message flood rate within the network, thus significantly reducing the message flood rate in the network, as well as the associated control plane cost. 
     Specifically, according to one or more embodiments of the disclosure as described in detail below, a reactive routing network may be dynamically divided into reactive routing network sub-domains that each include nodes with bounded route request (RREQ) scopes (e.g., search-domains) that are limited to a particular path length. In other words, a reactive routing network sub-domain includes a plurality of nodes, each of which has a search-domain with a limited number of surrounding nodes that may receive a RREQ from that particular node. A transit node may receive a RREQ from an originating node within a first reactive routing network sub-domain for a target node determined by the originating node to be beyond the bounded RREQ scope (i.e., search-domain) of the originating node. The transit node may then discover a route from the transit node to the target node, and return the route to the originating node. The discovered route may include at least one node in a second reactive routing network sub-domain, which is outside of the first reactive routing network sub-domain. In this manner, the transit node may establish a complete route between the originating node and the target node. In addition, according to one or more additional embodiments of the disclosure, a node within a reactive routing network may receive a segmentation message from a capable node (e.g., a transit node, a LBR, etc.), and in response, establish a bounded RREQ scope (e.g., a search-domain) for any RREQ originated by the node that is limited to a particular path length. As such, the node may forward RREQs to one or more transit nodes for any target node not identified by the node as being within the search-domain of the node. 
     Illustratively, the techniques described herein may be performed by hardware, software, and/or firmware, such as in accordance with the routing process  244 , which may contain computer executable instructions executed by the processor  220  (or independent processor of interfaces  210 ) to perform functions relating to the novel techniques described herein. For example, the techniques herein may be treated as extensions to conventional routing protocols, such as the various reactive routing protocols, and as such, may be processed by similar components understood in the art that execute those protocols, accordingly. 
     The techniques herein may dynamically divide a reactive routing network into reactive routing sub-domains based on observed message flood patterns within the network and/or based on network conditions (e.g., link/node congestion state). Advantageously, the reactive routing sub-domains of the disclosure may restrict flooding scope within the network as a whole by, for example, establishing a search-domain for each node within the reactive routing sub-domain that comprises a limited number of surrounding nodes that may receive a RREQ from that particular node. In the event that a node within a reactive routing sub-domain (e.g., an originating node) initiates a RREQ within its search-domain and is unable to identify a path to a desired target node, the originating node may then attempt to identify the target node with the aide of a transit node. For example, unicast or loose source routing may be used to reach out (or out-of-search-domain) target destinations via dynamically discovered points of transit (e.g., transit nodes), with a resulting decrease in control plane cost overhead. Additionally, capable nodes within the reactive routing network may serve to establish search-domain boundaries within the network, which may be based on control plane cost overhead due to flooded messages (e.g., network state) within the reactive routing network prior to reactive routing network division into reactive routing sub-domains. 
     According to the techniques herein, route discovery in a reactive routing network may be facilitated by the use of transit nodes. For example,  FIG. 3A  depicts a reactive routing network comprising eleven nodes, including a Root/LBR. Consider the situation in which node  13  needs to find a route in the network to node  43 . In a typical reactive routing network, node  13  will broadcast a probe/message (e.g., a RREQ) in order to discover a route to node  43 . Given this scenario, there are multiple approaches that may be taken for such route discovery:
         1) Node  43  may return all of the received messages/probes to node  13  with the recorded path (e.g., a route reply or “RREP”), and path selection may be performed by node  13  on the basis of the path cost, as indicated by the received messages/probes;   2) Node  43  may arm a timer upon receiving the first message/probe from node  13 , and once the timer has expired, node  43  may select the received message/probe with the “best” path according to the path cost; and/or   3) Node  43  may immediately return the first received message/probe to node  13  in order to avoid wasting time before sending the data packet, and may store the path cost for that particular message/probe and only return further messages/probes if their path cost is better then the path cost for the original message/probe by a particular value “X” (e.g., by X %).       

     Illustratively, once a route is discovered between node  13  (e.g., source/requestor) and node  43  (e.g., destination), for example  13 - 12 - 22 - 32 - 43  as shown in  FIG. 3B , the source may arm a timer T 1 , which establishes the period of time the route may be maintained. In another embodiment, the timer T 1  may also be aimed by intermediate nodes when a hop-by-hop routing protocol is used. After the expiration of timer TI, the route may be flushed and the route discovery process may occur again the next time a new path to node  43  is needed. It will be appreciated by the skilled artisan that increasing the value of T 1  may minimize the undesirable effect of flooding messages/probes at the risk of increasing the amount of states in the nodes (e.g., number of stored routed), but more importantly may increase the probability of using a stale route. 
     Operationally, the techniques herein may provide a software module referred to as a Distributed Intelligent Agent-Broadcast (DIA B), which may be hosted by a capable node/device (e.g., the LBR) within the reactive routing network or on a separate device, such as a network management server (NMS) or other management device. For example, the DIA-B may be encompassed by routing process  244  (see, e.g.,  FIG. 2 ). As shown in  FIG. 3C  (depicting a simplified view of an expanded network), the DIA-B may monitor the amount of control-plane traffic occurring in the network. In particular, the DIA-B may monitor the amount/volume of discovery messages/probes that are being flooded in the network. It is important to note that in the situation where the scope of particular messages/probes broadcast within the network is limited, such messages/probes may not reach the DIA-B. Consequently, the level of control plane traffic observed by the DIA-B may establish a lower bound for a particular network region when the discovery messages/probes are limited in scope. However, discovery messages/probes that are not limited in scope incur much more cost, and the DIA-B may monitor these messages/probes. Furthermore, the DIA-B may monitor the current state of the network by collecting information about a variety of network parameters such as, for example, traffic load, congestion, and the like, from various nodes within the network to either the LBR or a NMS. It is contemplated within the scope of the disclosure that link usage/congestion areas may be locally available at the LBR/DIA-B or may be obtained from the NMS/CIC (e.g., a central intelligence controller). 
     In addition, the techniques herein provide that all DIA-B processes, or other capable nodes/devices, may announce themselves as potential transit nodes. For example, DIA-B processes may self-identify as potential transit nodes via an IPv6 broadcast message(s), or via a routing protocol that may advertise their node capability such as, for example, a routing metric specified in IETF Internet Draft, entitled “Routing Metrics used for Path Calculation in Low Power and Lossy Networks” &lt;draft-ietf-roll-routing-metrics-19&gt; by Vasseur, et al. (Mar. 1, 2011 version) for the RPL protocol, or IS-IS node capability extensions in the event that ISIS may be used by the LBR on the core network backbone. 
     Operationally, the techniques herein provide a control plane overhead threshold value that may determine when routing region division should occur within the reactive routing network. For example, if the DIA-B process determines that the control plane overhead due to flooded messages/probes (e.g., RREQs) does not exceed the pre-defined threshold value, or that there are no congested areas in the network that would benefit from a reduction of the number of flooded messages/probes, then the DIA-B process may determine that no action is required with respect to routing region division. However, if the DIA-B process determines that the control plane overhead due to flooded messages/probes does exceed the pre-defined threshold value, then the DIA-B process may trigger dynamic division of the reactive routing network into one or more routing sub-domains via the following exemplary set of actions. 
     A capable node/device (e.g., an LBR, a NMS, a transit node, etc.) may perform a scoped flood of a new message, referred to herein as the Segmentation Message (e.g., an IPv6 message), for all destinations within a specific distance such that all nodes receiving the Segmentation Message comprise a reactive routing sub-domain. The Segmentation Message may cause nodes receiving the message to bound the scope of any subsequent route discovery messages (e.g., RREQs) flooded by such receiving nodes, e.g., by setting the TTL value of the flooded route discovery messages to specified path length “PL(i)”, as described below, which may create search-domains for each node within the reactive routing sub-domain. In this manner, the Segmentation Message may create a threshold at which the receiving node (e.g., a source node) may transition from a flooding protocol to a transit node transmission protocol when discovering routes within the network. For example, the Segmentation Message may establish a threshold level at which a source node may transition from a protocol of flooding a RREQ within the bounded scope of surrounding nodes/devices set by the Segmentation Message (i.e., a search-domain), to a protocol of transmitting messages directly to a transit node (e.g., the transit node that originated the Segmentation Message, or another transit node), which may then continue the search to complete the route request (if the transit node is not already aware of the intended target node of the route request). 
     Illustratively, direct transmission from the source node to the transit node may occur by unicasting or by “loose-hop” routing with the transit node set as the first next loose hop. In other words, a capable node may transmit a Segmentation Message to a subset of nodes/devices within a reactive routing network via a scoped flood (e.g., a sub-domain), and the Segmentation Message may then direct the subset of nodes/devices (i.e., the routing sub-domain) to use flooding to identify any target node ≦“X” hops away (e.g., PL(i)), which creates a search-domain, and if no RREP is received within “N” attempts, to then transmit that corresponding RREQ directly to a transit node via unicast or loose hop routing so that the transit node may continue the search for the target node. 
     Note that in one embodiment, the Segmentation Message may be unicast to any capable node (e.g., a LBR, a transit node, a NMS, etc.), which may then flood the Segmentation Message with a time-to-live (TTL) indicator (i.e., a scoped flood of the Segmentation Message). Advantageously, this approach may allow the capable node (e.g., the LBR/root or transit node) to divide the network into one or more routing sub-domains by delivering the Segmentation Message to a localized region of the network. In another embodiment, the Segmentation Message may be broadcast to all nodes in the network (e.g., from a central network management device), and may affect how all nodes in the network operate. In still another embodiment, the Segmentation Message may be unicast to individual nodes within the reactive routing network. It is contemplated within the scope of the disclosure that a routing sub-domain may, or may not, contain a transit node. 
     Illustratively,  FIG. 3C  depicts an expanded network in which the DIA-B may monitor the message/probe broadcast rate within the expanded network. If the DIA-B process determines that the level of flooding within the expanded network is too high, or that particular links within the expanded network (e.g.,  13 -LBR 2  or  23 - 24 ) are congested, the DIA-B process may dynamically signal one or more nodes (e.g., the Root/LBR) to advertise itself/themselves as a “transit node(s).” For example, as shown in  FIG. 3D , LBR 1  may self-identify as a transit node and begin broadcasting segmentation message (SM)  300 , which may cause dynamic division of the routing region of the expanded network into two or more routing sub-domains, as shown in  FIG. 3E . 
     Upon receiving the SM  300 , each node within the network may begin bounding the scope of any flooded message/probe (RREQ) by setting the TTL value of the packet to PL(i), effectively creating search-domains within the routing sub-domain. For example, if PL=3, then node  21  in the expanded network would not be able to find a direct route to node  25  using RREQ messages because it exceeds the hop threshold. Instead, a route from node  21  to note  25  may be established using the mechanism described below. 
     If a destination node cannot be reached within the source node&#39;s search-domain (e.g., no RREP packet has been received after “N” trials, where N≧1), then the source node may begin to use loose routing, with LBR 1  set as the first next loose hop, essentially, to let the transit node complete the unknown path to the destination/target node. If the route to the LBR 1  is known, then the source node may source route the packet with the last two entries listed as loose hops. For example, if the packet received by node  42  seeks a path to node  25 , the packet may carry the following source route:  42 - 32 - 22 - 12 -LBR 1 (L)- 25 (L) (where “(L)” indicates the ends of a loose hop). If the source node does not know the source route to the closest transit node, it may either send a message/probe to discover a path to the node or, if available, it may use a simple proactive DAG to provide hop-by-hop upward routing to the LBR of interest. Upon receiving such a loose route message/probe, LBR 1  may then add the next hop entry (e.g., LBR 2 ) in any of a variety of ways. For example, LBR 1  may multicast the RREQ to other transit nodes within the reactive routing network to determine whether any may be able to complete the route to the target node. If none of the queried transit nodes are able to complete the route (e.g., if the target node is not located within a transit node associated network sub-domain), LBR 1  may then flood the RREQ to the entire network to identify a route to the target node. 
     In the event that the target node is located within the LBR 2  network sub-domain, and LBR 2  knows the path to the target node, then LBR 2  may return the completed route to LBR 1 . However, if LBR 2  does not know the route to the target node, it may then initiate a local message/probe broadcast with the destination target node desired by the source node with, for example, a TTL value of PL( 2 ) (i.e., the value of the Path Length in its own search-domain). Upon receiving the reply (e.g., a RREP) from the destination node, the discovered path may be added to the RREP messages and sent back to the requesting LBR, which may, in turn, return the RREP to the source node with the fully discovered path  310 , for example,  42 - 32 - 22 - 12 -LBR 1 -LBR 2 - 14 - 25 , as shown in  FIG. 3F . 
     In addition, the LBRs may keep track of the number of identified loose routes so as to dynamically adjust the values of PL(i). Larger values of PL(i) may lead to wider search-domains and more optimal paths at the cost of increased broadcast domains. 
     Notably, the value of PL(i) may have a number of consequences, and may be chosen by the initiating LBR according to the presence of other LBRs to make sure that PL(i) (i being the search-domain) may be chosen so as to guarantee existence of a path between each pair of nodes in the network. For example, as described above, if LBR 2  sets the TTL value as PL( 1 )=4, then node  42  would not be able to establish a route to node  14 . In order to compensate for this scenario, LBR 2  may set the value of PL( 2 ) high enough to guarantee that a path will be found. 
     In view of the foregoing, one of skill in the art will appreciate that  FIGS. 3C-3F  represent simplified views of an exemplary extended network. For example,  FIG. 3G  depicts dynamic division of a reactive routing network into routing sub-domains in a more complex reactive routing network. As described above, a node within the reactive routing network may self-identify as a transit node (e.g., TN 1 )—upon its own determination or upon instruction from a management device—and broadcast an SM  300  with a PL( 1 ) value of, for example, PL( 1 )=2. The broadcast of SM  300  with a TTL of 2 may establish TN 1  routing sub-domain  315 , as shown in  FIG. 3G , in which each node may have a bounded RREQ scope of PL( 1 )=2. Accordingly, each node within TN 1  routing sub-domain  315  may have its own bounded RREQ scope search-domain  320 , as shown in  FIG. 3G  (for illustrative purposes, only one such search-domain is depicted for a specific node—shown as source node  309 ). Said differently, each node within a range of the SM  300  (e.g., two hops away from the transit node) may each have their own search boundary that is PL( 1 ) away (e.g., two hops away from the particular node). As described above, if source node  309  initiates a local-scoped flood of a RREQ for target node  311  within bounded scope search-domain  320 , and no RREP is received within “N” attempts, then source node  309  may unicast the RREQ directly to TN 1  (or it may switch to loose routing with TN 1  set as the first next loose hop). 
     It is contemplated within the scope of the disclosure that reactive routing network sub-domains may, or may not, overlap. As shown in  FIG. 3H , TN 2  may identify as a transit node and broadcast a SM  300  with PL( 2 )(not shown) to generate TN 2  routing sub-domain  325 , which may be (though need not be) approximately the same size as, and overlap with, TNT routing sub-domain  315 . Such overlapping sub-domains do not create an issue for the techniques herein because of the overlapping nature of the bounded RREQ scope search-domains for each node within TN 1  routing sub-domain  315  and/or TN 2  routing sub-domain  320 . In other words, due to their bounded RREQ scope search-domains and geo-spatial location within the reactive routing network, some nodes within TN 1  routing sub-domain  315  may be able to directly query some nodes in TN 2  routing sub-domain  320 , but not others. For the latter nodes, the RREQ may transit via TN 1  to reach the desired target node in TN 2  routing sub-domain  320  by any of the methods described above. Note that should a node receive two segmentation messages, that node may simply select one of the corresponding transit nodes, or may load-balance between the two. 
     In addition, as described above, the techniques herein provide that a source node within a particular routing sub-domain may route a RREQ to one or more different transit node(s). For example,  FIG. 31  depicts a reactive routing network in which TN 1  has initiated a scoped broadcast of a Segmentation Message that establishes TN 1  routing sub-domain  330 . However, the Segmentation Message may establish bounded search scope search-domains for nodes within TN 1  routing sub-domain  330  that may be completely different from the scope of its own initial broadcast (i.e., the routing sub-domain). In this context, a source node within TN 1  routing sub-domain  330  may be positioned within the network such that a different transit node (e.g., TN 2 ) is closer to the source node than TN 1 . In this case, if source node  309  initiates a local-scoped flood of a RREQ for target node  311  within bounded scope search-domain  335 , and no RREP is received within “N” attempts, then source node  309  may unicast the RREQ directly to TN 2  instead of TN 1 . In one embodiment, the source node  309  may keep track of the “closest” transit node by any of a variety of distance metrics (e.g., hop count, reliability, latency, etc.). In one embodiment, this distance metric may be obtained from the Segmentation Message. In another embodiment, the node may flood a RREQ to discover one or more transit nodes. Although such a transit node discovery method would initiate a network flood, the flood only occurs to the extent necessary to find suitable transit nodes, to which unicast RREQs may then be sent to discover routes to other target devices. 
     In addition, the techniques herein also provide that TN 1  may directly query specific “linked” transit nodes within the dynamically divided reactive routing network in order to complete route discovery. For example, as shown in  FIG. 3J , TN 1  may maintain proactive DAGs to specific “linked” transit nodes within the network such as, for example, TN 2 , TN 3 , and TN 4 . In order to query “non-linked” transit nodes such as, for example, TN 5 , or nodes that are not resident within a transit node-associated network sub-domain (e.g., slant hashed nodes in  FIG. 33 ), TN 1  may flood the RREQ of the originating node to the entire network. 
     The techniques herein provide a significant increase in efficiency and decrease in control plane overhead because the bounded RREQ scope search-domains and the ability of transit nodes to efficiently complete route discover may significantly decrease overall network traffic. In addition, even if it is necessary for a particular transit node (e.g., LBR 1 /TN 1 ) to initiate a network flood to identify a route to a target node, the techniques herein may allow that discovered route to remain available for other nodes within the LBR 1  sub-domain looking to reach the same target node, which may prevent additional network floods. 
       FIG. 4  illustrates an example simplified procedure  400  for dynamic division of a reactive routing network into reactive routing sub-domains in accordance with one or more embodiments described herein, particularly from the perspective of a transit node. The procedure  400  may start at step  405 , and continue to step  410  where, as described above, a transit node in a first reactive routing network sub-domain may receive a RREQ from an originating node within the first reactive routing network sub-domain for a target node determined by the originating node to be beyond the bounded RREQ scope search-domain of the originating node. As shown in step  415 , the transit node may then discover a route from the transit node to the target node. As shown in step  420 , the transit node may then return the route to the originating node. In this manner, the transit node may establish a complete route between the originating node and the target node, and then the procedure  400  may illustratively end at step  425 . In other words, the transit node may act as an intermediary between the first reactive routing network sub-domain and target nodes within other reactive routing network sub-domains, which dramatically decreases the control overhead required to discover and establish complete paths between an originating node and a target node within a reactive routing network. 
     Similarly,  FIG. 5  illustrates an example simplified procedure  500  for dynamic division of a reactive routing network into reactive routing sub-domains in accordance with one or more embodiments described herein, particularly from the perspective of a requesting node. The procedure  500  may start at step  505 , and continue to step  510  where, as described above, a node within a reactive routing network may receive a segmentation message from an originating capable node (e.g., a transit node, a LBR, etc.). As shown in step  515 , the segmentation message may function to establish a bounded RREQ scope for any RREQ originated by the node that is limited to a particular path length, effectively establishing a search-domain centered around the node. As shown in step  520 , when the node originates a RREQ within the predetermined bounded scope of the search-domain and fails to identify the desired target node, the node may then forward the RREQ to a receiving transit node, and then the procedure  500  may illustratively end at step  525 . In this manner, the originating node initiates an efficient bounded scope search for the target within the search-domain, and if this search is unsuccessful the originating node forwards the RREQ to a receiving transit node, which may or may not be within the first reactive routing network sub-domain, to act as an intermediary to complete the route request. Accordingly, the techniques herein may dramatically decreases the control overhead required to discover and establish complete paths between and originating node and a target node. 
     It should be noted that while certain steps within procedures  400  and  500  may be optional as described above, the steps shown in  FIGS. 4 and 5  are merely examples for illustration, and certain other steps may be included or excluded as desired. Further, while a particular order of the steps is shown, this ordering is merely illustrative, and any suitable arrangement of the steps may be utilized without departing from the scope of the embodiments herein. Moreover, while procedures  400 - 500  are described separately, certain steps from each procedure may be incorporated into each other procedure, and the procedures are not meant to be mutually exclusive. 
     The techniques described herein, therefore, provide for dynamic division of reactive routing networks into reactive routing sub-domains in order to control/minimize flooding, which provides increased scalability for reactive routing networks. By using the transit nodes as a bridge to help reach the final destination node, the techniques herein may reduce congestion in reactive routing networks. In particular, the techniques herein increase scalability both for an increase in the number of nodes in a network, and for small networks as the number of active P2P flows in the network increases. 
     While there have been shown and described illustrative embodiments of techniques for use with reactive routing in communication networks, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the embodiments herein. For example, the embodiments have been shown and described herein with relation to LLNs. However, the embodiments in their broader sense are not as limited, and may, in fact, be used with other types of networks, regardless of whether they are considered constrained. In addition, while certain protocols are shown, other suitable protocols may be used, accordingly. 
     The foregoing description has been directed to specific embodiments. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. For instance, it is expressly contemplated that the components and/or elements described herein can be implemented as software being stored on a tangible (non-transitory) computer-readable medium (e.g., disks/CDs/RAM/EEPROM/etc.) having program instructions executing on a computer, hardware, firmware, or a combination thereof Accordingly this description is to be taken only by way of example and not to otherwise limit the scope of the embodiments herein. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the embodiments herein.