In one embodiment, a node in a shared-media communication network may determine a first directed acyclic graph (DAG) topology, wherein the first DAG topology has a particular direction. The node may determine a second DAG topology in the shared-media communication network based on the first DAG topology. The second DAG topology may share the particular direction of the first DAG topology, to prevent loops between the first and the second DAG topologies.

TECHNICAL FIELD

The present disclosure relates generally to computer networks, and, more particularly, to multi-topology routing.

BACKGROUND

Low power and Lossy Networks (LLNs), e.g., sensor networks, have a myriad of applications, such as Smart Grid and Smart Cities. Various challenges are presented with LLNs, such as lossy links, low bandwidth, battery operation, low memory and/or processing capability, etc., particularly given that LLNs are generally a shared-media communication network. One example routing solution to LLN challenges is a protocol called Routing Protocol for LLNs or “RPL,” which is a distance vector routing protocol that builds a Destination Oriented Directed Acyclic Graph (DODAG, or simply DAG) in addition to a set of features to bound the control traffic, support local (and slow) repair, etc. The RPL architecture provides a flexible method by which each node performs DODAG discovery, construction, and maintenance.

Existing solutions include maintaining multiple next-hop routes to improve reliability and latency in the shared-media communication network. Specifically, in the existing solutions a single routing topology utilizing a common set of metrics (e.g., hop count or estimated transmission count, “ETX”) and constraints (e.g., received signal strength indicator, “RSSI”, threshold to promote link stability) may be built. However, these solutions require devices to choose or at least balance between a topology emphasizing reduced number of hops or a topology increasing route stability because metrics and constraints are often competing factors when building a routing topology.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

According to one or more embodiments of the disclosure, a node in a shared-media communication network may determine a first directed acyclic graph (DAG) topology, wherein the first DAG topology has a particular direction. The node may then determine a second DAG topology in the shared-media communication network based on the first DAG topology. The second DAG topology may share the particular direction of the first DAG topology, to prevent loops between the first and the second DAG topologies.

Description

FIG. 1is a schematic block diagram of an example computer network100illustratively comprising nodes/devices200(e.g., labeled as shown, “root,” “11,” “12,” . . . “45,” and described inFIG. 2below) interconnected by various methods of communication. For instance, the links105may be wired links or shared-media (e.g., wireless links, PLC links, etc.) where certain nodes200, such as, e.g., routers, sensors, computers, etc., may be in communication with other nodes200, 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 network100is merely an example illustration that is not meant to limit the disclosure.

Data packets140(e.g., traffic and/or messages sent between the devices/nodes) may be exchanged among the nodes/devices of the computer network100using 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. 2is a schematic block diagram of an example node/device200that may be used with one or more embodiments described herein, e.g., as any of the nodes shown inFIG. 1above. The device may comprise one or more network interfaces210(e.g., wired, wireless, PLC, etc.), at least one processor220, and a memory240interconnected by a system bus250, as well as a power supply260(e.g., battery, plug-in, etc.).

The network interface(s)210contain the mechanical, electrical, and signaling circuitry for communicating data over links105coupled to the network100. 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 connections210, e.g., wireless and wired/physical connections, and that the view herein is merely for illustration. Also, while the network interface210is shown separately from power supply260, for PLC the network interface210may communicate through the power supply260, 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 memory240comprises a plurality of storage locations that are addressable by the processor220and the network interfaces210for 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 processor220may comprise hardware elements or hardware logic adapted to execute the software programs and manipulate the data structures245. An operating system242, portions of which are typically resident in memory240and 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 routing process/services244, a directed acyclic graph (DAG) process246, and an illustrative routing topology determination process248, as described herein. Note that while dynamic multi-path process248is shown in centralized memory240, alternative embodiments provide for the process (or portions of the process) to be specifically operated within the network interfaces210(process “248a”).

Furthermore, Low power and Lossy Networks (LLNs), e.g., certain sensor networks, may be used in a myriad of applications such as for “Smart Grid” and “Smart Cities.” A number of challenges in LLNs have been presented, such as:

1) Links are generally lossy, such that a Packet Delivery Rate/Ratio (PDR) can dramatically vary due to various sources of interferences, e.g., considerably affecting the bit error rate (BER);

2) Links are generally low bandwidth, such that control plane traffic must generally be bounded and negligible compared to the low rate data traffic;

3) There are a number of use cases that require specifying a set of link and node metrics, some of them being dynamic, thus requiring specific smoothing functions to avoid routing instability, considerably draining bandwidth and energy;

4) Constraint-routing may be required by some applications, e.g., to establish routing paths that will avoid non-encrypted links, nodes running low on energy, etc.;

5) Scale of the networks may become very large, e.g., on the order of several thousands to millions of nodes; and

6) Nodes may be constrained with a low memory, a reduced processing capability, a low power supply (e.g., battery).

In other words, LLNs are a class of network 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 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 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 (pronounced “ripple”) may generally be described as a distance vector routing protocol that builds a Directed Acyclic Graph (DAG) for use in routing traffic/packets140, in addition to defining a set of features to bound the control traffic, support repair, etc. Notably, as may be appreciated by those skilled in the art, RPL also supports Multi-Topology-Routing (MTR), whereby multiple DAGs are built to carry traffic according to individual requirements.

A DAG is a directed graph having the property that all edges (and/or vertices) are oriented in such a way that no cycles (loops) are supposed to exist. All edges are contained in paths oriented toward and terminating at one or more root nodes (e.g., “clusterheads” or “sinks”), often to interconnect the devices of the DAG with a larger infrastructure, such as the Internet, a wide area network, or other domain. In addition, a Destination Oriented DAG (DODAG) is a DAG rooted at a single destination, i.e., at a single DAG root with no outgoing edges. A “parent” of a particular node within a DAG an immediate successor of the particular node on a path towards the DAG root, such that the parent has a lower “rank” than the particular node itself, where the rank of a node identifies the node's position with respect to a DAG root (e.g., the farther away a node is from a root, the higher is the rank of that node). Further, in certain embodiments, a sibling of a node within a DAG may be defined as any neighboring node which is located at the same rank within a DAG. Note that siblings do not necessarily share a common parent, and routes between siblings are generally not part of a DAG since there is no forward progress (their rank is the same). Note also that a tree is a kind of DAG, where each device/node in the DAG generally has one parent or one preferred parent.

DAGs may generally be built (e.g., by DAG process246) based on an Objective Function (OF). The role of the Objective Function is generally to specify rules on how to build the DAG (e.g. number of parents, backup parents, etc.).

In addition, one or more metrics/constraints may be advertised by the routing protocol to optimize the DAG against. Also, the routing protocol allows for including an optional set of constraints to compute a constrained path, such as if a link or a node does not satisfy a required constraint, it is “pruned” from the candidate list when computing the best path. (Alternatively, the constraints and metrics may be separated from the OF.) Additionally, the routing protocol may include a “goal” that defines a host or set of hosts, such as a host serving as a data collection point, or a gateway providing connectivity to an external infrastructure, where a DAG's primary objective is to have the devices within the DAG be able to reach the goal. In the case where a node is unable to comply with an objective function or does not understand or support the advertised metric, it may be configured to join a DAG as a leaf node. As used herein, the various metrics, constraints, policies, etc., are considered “DAG parameters.”

Illustratively, example metrics used to select paths (e.g., preferred parents) may comprise cost, delay, latency, bandwidth, expected transmission count (ETX), etc., while example constraints that may be placed on the route selection may comprise various reliability thresholds, restrictions on battery operation, multipath diversity, bandwidth requirements, transmission types (e.g., wired, wireless, etc.). The OF may provide rules defining the load balancing requirements, such as a number of selected parents (e.g., single parent trees or multi-parent DAGs). Notably, an example for how routing metrics and constraints may be obtained may be found in an IETF RFC, entitled “Routing Metrics used for Path Calculation in Low Power and Lossy Networks” <RFC 6551> by Vasseur, et al. (March 2012 version). Further, an example OF (e.g., a default OF) may be found in an IETF RFC, entitled “RPL Objective Function 0” <RFC 6552> by Thubert (March 2012 version) and “The Minimum Rank Objective Function with Hysteresis” <RFC 6719> by O. Gnawali et al. (September 2012 version).

The process of building a DAG may utilize a discovery mechanism to build a logical representation of the network, and route dissemination to establish state within the network so that routers know how to forward packets toward their ultimate destination. Note that a “router” refers to a device that can forward as well as generate traffic, while a “host” refers to a device that can generate but does not forward traffic. Also, a “leaf” may be used to generally describe a non-router that is connected to a DAG by one or more routers, but cannot itself forward traffic received on the DAG to another router on the DAG. Control messages may be transmitted among the devices within the network for discovery and route dissemination when building a DAG.

According to the illustrative RPL protocol, a DODAG Information Object (DIO) is a type of DAG discovery message that carries information that allows a node to discover a RPL Instance, learn its configuration parameters, select a DODAG parent set, and maintain the upward routing topology. In addition, a Destination Advertisement Object (DAO) is a type of DAG discovery reply message that conveys destination information upwards along the DODAG so that a DODAG root (and other intermediate nodes) can provision downward routes. A DAO message includes prefix information to identify destinations, a capability to record routes in support of source routing, and information to determine the freshness of a particular advertisement. Notably, “upward” or “up” paths are routes that lead in the direction from leaf nodes towards DAG roots, e.g., following the orientation of the edges within the DAG. Conversely, “downward” or “down” paths are routes that lead in the direction from DAG roots towards leaf nodes, e.g., generally going in the opposite direction to the upward messages within the DAG.

Generally, a DAG discovery request (e.g., DIO) message is transmitted from the root device(s) of the DAG downward toward the leaves, informing each successive receiving device how to reach the root device (that is, from where the request is received is generally the direction of the root). Accordingly, a DAG is created in the upward direction toward the root device. The DAG discovery reply (e.g., DAO) may then be returned from the leaves to the root device(s) (unless unnecessary, such as for UP flows only), informing each successive receiving device in the other direction how to reach the leaves for downward routes. Nodes that are capable of maintaining routing state may aggregate routes from DAO messages that they receive before transmitting a DAO message. Nodes that are not capable of maintaining routing state, however, may attach a next-hop parent address. The DAO message is then sent directly to the DODAG root that can in turn build the topology and locally compute downward routes to all nodes in the DODAG. Such nodes are then reachable using source routing techniques over regions of the DAG that are incapable of storing downward routing state. In addition, RPL also specifies a message called the DIS (DODAG Information Solicitation) message that is sent under specific circumstances so as to discover DAG neighbors and join a DAG or restore connectivity.

FIG. 3illustrates an example simplified DAG that may be created, e.g., through the techniques described above, within network100ofFIG. 1. For instance, certain links105may be selected for each node to communicate with a particular parent (and thus, in the reverse, to communicate with a child, if one exists). These selected links form the DAG310(shown as bolded lines), which extends from the root node toward one or more leaf nodes (nodes without children). Traffic/packets140(shown inFIG. 1) may then traverse the DAG310in either the upward direction toward the root or downward toward the leaf nodes, particularly as described herein.

As noted above, existing LLN solutions typically form a single routing topology (e.g., a single DAG). Thus, routing is performed over a topology using either links that reduce latency and channel utilization (e.g., “long” links) or links that increase link stability and robustness (e.g., “short” links). Furthermore, these solutions apply a minimum link margin constraint threshold in selecting next-hop routes. For example, in a connected grid mesh solution, only routes having a received signal strength indicator (RSSI) of 10 dBm greater than the transceiver's rated sensitivity are considered. In this particular solution, the 10 dBm threshold is based on observed behaviors with varying environmental conditions. In addition, the link may be utilized for forwarding until the ETX reaches a predetermined limit.

However, enforcing a hysteresis on link margin constraints limits the network from utilizing links with reduced hop count (e.g., long links) when they are available. Lowering the hysteresis threshold promotes the use of links with reduced hop count but also increases the risk of link instability. In other words, links with a low link margin constraint may become unstable.

One option available to create multiple routing topologies is known as multi-topology routing (MTR), where each topology is created independently. For example, a first and second topology may be built independently with different hysteresis thresholds. Based on the example above, for instance, devices in the network100may build one routing topology based on link stability and a second routing topology, independent of the first, based on a reduced hop count. However, once a routing topology is determined for forwarding given traffic, switching to the other topology should not be allowed for that traffic. In particular, since the topologies are created independently (i.e., may choose different paths and different directions within the network), switching between the topologies may result in routing loops (cycles).

The techniques herein, however, provide a mechanism that allows a device (e.g., a node) in a shared-media communication network to determine multiple DAG topologies in a manner that prevent loops in multi topology routing (MTR), particularly that use link stability (e.g., a stable DAG topology) and reduced hop count (e.g., a low latency DAG topology). For instance, to prevent loops in MTR, one topology (e.g., the reduced hop count DAG topology) may be built based on a previously created topology (e.g., the DAG topology emphasizing link stability), thereby, promoting the same direction of forwarding. For example, in an illustrative embodiment as described below, the network may first build a stable DAG topology by considering only next-hop routes that provide a link margin above a predetermined threshold constraint. The network may then collectively build a second DAG topology by considering next-hop routes with a relaxed link margin constraint and constraining routes to follow the particular direction of the first DAG topology. The link margin threshold for the first DAG topology (e.g., the stable DAG topology) may be an adaptive parameter applied to the entire network and controlled by a DAG root or may be a parameter locally adjusted by each device. Furthermore, since the second DAG topology in constrained to follow the particular direction of the first DAG topology, the combination of the first and the second DAG topologies does not form any routing loops. Thus, the devices may forward packets over the first or the second DAG topology, interchangeably, without a risk of encountering forwarding loops.

Illustratively, the techniques described herein may be performed by hardware, software, and/or firmware, such as in accordance with the routing topology determination process248/248a, which may contain computer executable instructions executed by the processor220(or independent processor of interfaces210) to perform functions relating to the techniques described herein, e.g., in conjunction with routing process244and/or DAG process246. For example, the techniques herein may be treated as extensions to conventional protocols, such as the various routing topology creation (and utilization) protocols, and as such, may be processed by similar components understood in the art that execute those protocols, accordingly.

Operationally, the techniques herein relate to multi-topology routing (MTR) in a shared-media communication network allowing nodes to forward over a stable link topology or a reduced hop count (e.g., low latency) topology. In particular, the first topology may be built by considering only highly-stable links or considering only next-hop routes that provide a link margin above a predetermined threshold constraint. The second topology may then be built based on the first topology, wherein both topologies follow the same particular direction. Thus, the two topologies together do not form any forwarding loops or cycles. Notably, though a certain ordering of topology creation is described herein (e.g., stable links then low latency links), such an ordering is merely one example embodiment, and any desired metric and/or constraint may be used to create the first and second topologies, so long as the subsequently created topology is based on the previously created topology, as described herein.

For example, in RPL terminology, a device may construct a first DAG topology (e.g., a stable DAG topology) and may use the first DAG topology to constrain the structure of a second DAG topology (e.g., a low-latency DAG topology). Furthermore, although conventional multi-topology routing may require multiple RPL instances, these topologies are not dependent on each other. In other words, the conventional combination of multiple links across multiple RPL instances may form loops in the network. However, the techniques herein provide for a first DAG topology that is used to constrain the construction of a second DAG topology, thus preventing forwarding loops (i.e., the topologies are required to head in the same direction—e.g., toward the root). Although the techniques herein may apply to RPL, they are not limited thereto and may be applied to other multi-topology routing protocols.

In one embodiment, as shown above inFIG. 3, the network (e.g., devices200collectively) may build a first DAG topology310emphasizing link stability by considering only next-hop routes that provide a link margin above a predetermined threshold constraint. The link margin constraint may be a hysteresis threshold to ensure that the routes used to build the first DAG topology310may be robust for a predetermined duration. Additionally, in building the first DAG topology310, the link margin constraint may be used to minimize the path cost. Moreover, during or after the building of the first DAG topology310, the node may assign a DAG rank (as shown inFIG. 3as the first digit of the node numbers, e.g., rank “1” for nodes11,12, and13, rank “2” for nodes22,23, and24, etc.) corresponding to a topological distance to a root node of the first DAG topology310(e.g., as in RPL). Notably, in one embodiment, nodes may advertise a newly defined path metric indicating the smallest link margin constraint chosen by a node along a particular path.

According to the techniques herein, as shown inFIG. 4, the shared-media communication network may build a second DAG topology415emphasizing reduced hop count (e.g., long range links) by considering next-hop routes with a relaxed link margin constraint. Specifically, however, the constraint in the second topology also requires selecting a parent in the second DAG topology415to have a rank in the first DAG topology310that is less than or equal to the rank of a first parent in the first DAG topology310. In this manner, the second/subsequent routing topology is built while ensuring that the first and the second DAG topologies follow the same direction to prevent forwarding loops.

Note that in RPL, the second DAG topology415may be a second RPL instance with a different set of metrics and constraints. Alternatively, the metric may remain the same as the first DAG310(e.g., to minimize ETX) while the link margin constraint may be relaxed. Furthermore, as illustrated inFIG. 4, the second DAG topology415is not required to use (nor prevented from using) the links selected by the first DAG topology310. That is, the second DAG topology may utilize the topology ranks determined in the first DAG topology to determine a route to the destination while ensuring no forwarding loops exist between the first and the second DAG topologies.

In another embodiment, to ensure connectivity in the network, the node may send a message to a management node indicating insufficient next-hop routes that provide a link margin above a predetermined threshold constraint. The node may then receive a response message from a management node to relax the link margin constraint. In other words, the link margin constraint may be a network wide parameter which may be advertised by a DAG root, for example, in a RPL Metric Container within a RPL DIO message. In response to the network wide parameter, RPL devices may learn of a disconnected device by receiving the DIO message. Additionally, the RPL devices may send a message to the RPL Root notifying the Root of the disconnected device using a RPL DAO message. In response to the RPL DAO message, the RPL root may relax the link margin constraint advertised in the DIO message to promote connectivity across all the RPL devices. Notably, relaxing the link margin constraint may increase connectivity and simultaneously may decrease link stability within the network. Additionally, RPL is merely an example embodiment, and the techniques herein are not limited thereto and may be applied to other distance vector routing protocols.

In yet another embodiment, to ensure connectivity in the network, the link margin constraint may be adjusted by each node. For example, when a node determines insufficient next-hop routes in a first DAG topology (e.g., a stable DAG topology), the node may locally adjust its link margin constraint. For example, a node may increase the link margin constraint in response to determining a high ETX. Alternatively, the node may tighten the link margin constraint when the node determines a sufficient number of next-hop routes satisfying the link margin constraint.

In one embodiment herein, the nodes in the shared-media communication network may adjust one or both of the determined DAG topologies based on a determined delay in the network. For example, an end-node in the network may forward a time stamped probe along the first and the second DAG topologies. In response to receiving the probe, a DAG Root (e.g., a field area router) may compare the delays in the first DAG topology to the second DAG topology. The nodes may receive a message indicating the determined delay and in response to receiving the message, may adjust the built topologies and the link margin constraint accordingly.

As shown inFIGS. 5A-C, after the multiple DAG topologies have been determined, a node in the shared-media communication network may forward a message (e.g., a data packet140) using the determined DAG topologies as described herein. Specifically, nodes may forward the messages over one topology or the other based on changing network conditions. For example, in one embodiment the nodes may forward the messages over the second DAG topology415(e.g., the low latency DAG topology), emphasizing reduced hop count, until the link quality falls below a predetermined threshold. The link quality may be a link margin constraint, a signal strength (e.g., RSSI), ETX, or the like. In response to the link quality falling below the predetermined threshold, the node may switch topologies and begin to forward the message over the first DAG topology emphasizing link stability. In other words, the node may attempt to forward the message to the destination using fewer hops and transmissions than on a route emphasizing link stability until the links on the determined DAG topology fall below the predetermined threshold.

As shown inFIG. 5A, for instance, a source node43may initiate traffic (520) on the second topology415to node22, but node22may be experiencing a low quality on the low latency link directly to the root node. As such, node22may switch topologies to the stable link to node11to continue forwarding the traffic520to the root over more stable links, without fear of generating any looping/cycles of the traffic.

In another example, as illustrated inFIG. 5B, the nodes may forward the messages (525) over the second DAG topology415until the link quality falls below a predetermined threshold. In particular, as shown inFIG. 5B, source node44initiates the traffic525on the low latency topology415to node33, but then node33switches the forwarding to the stable topology310to node22. Node22, then, may switch back to forwarding over the second DAG topology415if its link quality on the second DAG topology above the predetermined threshold.

As yet another example as illustrated inFIG. 5C, the nodes may forward the message (530) over the first DAG topology310when initially the link quality is below the predetermined threshold along the path, but when a node is reached with a link quality above the predetermined threshold (e.g., node22) the traffic may switch topologies and use the second DAG topology415.

In other words, each node along the path of the traffic may make an independent decision of which topology to use, on a real-time per-packet basis, since there are no loops created between the two topologies according to the techniques herein. Notably, since alternating between topologies on a per-packet basis for multi-packet flows may result in out-of-order packet reception, various known techniques may be used to re-order the packets upon reception at the destination node (e.g., the root node). For example, assuming inFIG. 5Athat node22sends a first packet to node11on the stable topology, and a second packet of the same flow/stream directly to the root node on the low latency topology, the first packet might arrive at the root from node11after the second packet has already arrived directly from node22.

Notably, in another embodiment, the node may forward the message over both the first DAG topology and the second DAG topology, thereby utilizing the topology links in parallel versus sequentially and reducing delay in reaching the destination. However, when forwarding the message over both the first and second DAG topologies in parallel, the risk of packet duplication increases.

FIG. 6illustrates an example simplified procedure600for cycle-free multi-topology routing in a shared-media communication network in accordance with one or more embodiments described herein. The procedure600may start at step605, and continues to step610, where, as described in greater detail above, a node in the shared-media communication network may determine a first DAG topology, e.g., that emphasizes link stability. As shown in step615, the node may determine a second DAG topology that is based on the first DAG topology, e.g., that emphasizes low latency/hop count. Based on the determined topologies, the node may forward a message in step620, which is generally described in greater detail inFIG. 7below. The procedure illustratively ends in step625.

As noted,FIG. 7illustrates an example simplified procedure700of step620inFIG. 6, wherein the node may forward a message over the determined DAG topology. The procedure700may start at step705, and continues to step710, where, as described above, the node may forward a message over the second (e.g., preferred) DAG topology. As shown in step715, the node may determine whether the preference outweighs the issues of using the second DAG topology link, e.g., whether its link quality is below a predetermined threshold. Illustratively, when the link quality is not below the predetermined threshold, as shown in step720, the node may continue to forward the message over the second DAG topology and the procedure may then illustratively end in step730(repeating for each packet received at the node). Alternatively, when the link quality is below the predetermined threshold (or other metric is not being satisfies), as shown in step725, the node may switch the DAG topology and forward the message over the first DAG topology, and the procedure may then illustratively end in step730.

The techniques described herein, therefore, provide for loop-free multi-topology routing in a shared-media communication network. In particular, the techniques herein allow for switching between a first DAG topology, illustratively emphasizing link stability, and a second DAG topology, illustratively emphasizing reduced hop count. Thus, once a DAG topology is determined, that particular path (selected topology) need not be maintained to reach a destination. Furthermore, the techniques herein allow a device to forward messages over a reduced hop count DAG topology (e.g., low latency DAG topology) until a link quality falls below a predetermined threshold, at which point the device may switch to the DAG topology emphasizing link stability without risk of forwarding loops.

In particular, the techniques herein provide multi-hop routing wherein either maximum transmission range is considered or link stability over time is considered depending on link quality. Increasing the transmission range decreases the number of hops, forwarding delays, and transmission to reach a final destination, thereby minimizing communication latency and channel utilization. On the other hand, when link quality decreases link margin constraints are important to consider. Increasing link margin constraints at each hop increases the overall link quality and robustness, thereby allowing a device to be more tolerable against temporal changes in interference, physical obstructions, and propagation characteristics of the physical media. Additionally, increasing the link margin constraint of each hop decreases the ETX across each individual link.

While there have been shown and described illustrative embodiments that provide for determining and utilizing multiple DAG topologies in a shared-media communication network, 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 and the RPL protocol. However, the embodiments in their broader sense are not as limited, and may, in fact, be used with other types of shared-media networks and/or protocols (e.g., wireless). In addition, while the techniques generally describe initiation and determinations by a node in the shared-media communication network, a network management system/server (NMS) may also be used to provide intelligence to the network functions described herein, such that the NMS determines the first and the second DAG topologies and informs the nodes in the network of the determined DAG topologies.