The Lightweight On-demand Ad hoc Distance-vector Routing Protocol – Next Generation (LOADng) is a routing protocol, derived from AODV and extended for use in Mobile Ad hoc NETworks (MANETs) and Low-power and Lossy Networks (LLNs).
The LOADng control messages are carried by way of the Generalized MANET Packet/Message Format (RFC5444). Using the generalized message format, control messages can include TLV (Type-Length-Value) elements, permitting protocol extensions to be easily developed. LOADng supports routing using arbitrary additive metrics, which can be specified as extensions to this protocol.
LOADng has been ratified by ITU-T recommendation “G9903: Narrowband orthogonal frequency division multiplexing power line communication transceivers for G3-PLC networks” and has been deployed in G3-PLC Smart Grid.
Basic Route Discovery – Route Requests & Route Replies
As a reactive protocol, the basic operations of LOADng include:
- generation of Route Requests (RREQs) by a LOADng Router (originator) for discovering a route to a destination,
- forwarding of such RREQs until they reach the destination LOADng Router,
- generation of Route Replies (RREPs) upon receipt of an RREQ by the indicated destination, and
- unicast hop-by-hop forwarding of these RREPs towards the originator.
Figure 1 gives an example of RREQ message flooding. Router S initiates a route discovery to Router D. The RREQ message generated by Router S is retransmitted by other routers and flooded in the network until it reaches the destination router D. All the intermediate routers will set up a “reverse route” to Router S.
Please note that LOADng prohibits the intermediate route reply, i.e., even if the intermediate router has a route to the destination, it MUST NOT reply to the RREQ message, but simply forward the RREQ message.
Figure 2 shows how unicast RREP message forwarding works. When the RREQ message reaches the destination router D, D will generate an RREP message. The RREP is unicast to the route discovery originator S following the reverse route established during the RREQ flooding. All the routers receiving the RREP message will establish a “forward route” to Router D.
If a route is detected to be broken, e.g. if forwarding of a data packet to the recorded next hop on the route towards the intended destination is detected to fail, a Route Error (RERR) message is returned to the originator of that data packet to inform the originator about the route breakage.
LOADng specifies a slim core that supports basic point-to-point route establishment that is suitable for common scenarios. To improve the protocol performance in special scenarios, such as data collection or existence of extremely unreliable links, different extensions are developed.
Smart Route Request
In some network types, such as sensor networks, it is common to have sensor-to-root (multipoint-to-point — or MP2P) traffic. While eliminating intermediate RREPs can reduce the size of control message and simplify the protocol process, the side effect of blindly flooding RREQ cannot be ignored in this kind of scenarios.
Smart Route Request is thus proposed to replace intermediate route reply while retaining the loop-freedom nature and security mechanism of LOADng. When Smart Route Request is used, if an intermediate LOADng router has already a routing entry to the destination, it will retransmit the RREQ message to the destination by unicast, instead of multicast. More details about the Smart Route Request can be found at:
Expanding Ring Search
The expanding ring search aims at reducing the flooding area of RREQ messages so as to reduce the message overhead. A router will at first send an RREQ with a reduced TTL (Time-To-Live) — causing the RREQ to not be flooded through the entire network, but only up to a limited distance. If the destination sought receives the RREQ, or an intermediate router has a path to the sought destination, an RREP (possibly intermediate/gratuitous) is generated and a network-wide flooding is avoided. If no RREP is received by the originator in expected delay, another RREQ message is, after a brief delay, generated with increased TTL to eventually cover the entire network. More details about the expanding ring search can be found at:
Collection Tree Protocol
The point-to-point traffic pattern supported by LOADng matches the basic traffic model of the Internet. However, in many deployments of LLNs, another important traffic pattern, called sensor-to-root, or multipoint-to-point, exists. In such traffic scenarios, there is one or more devices that plays the role as “root” — data sink for all traffic — and where all the other devices in the network communicate with the root. If paths from all the other devices to the root are required, it is more efficient to build a “collection tree” (i.e., a directed graph in which all edges are oriented toward and terminate at one root router) and to discover and maintain the set of point-to-point routes from all other routers to that “root”. The collection tree extension aims at building bi-directional routes between the root router and all other routers. More details about the collection tree extension can be found at:
Depth First Forwarding
Depth-First Forwarding in Unreliable Networks (DFF) is an experimental data forwarding standard by the IETF, which proposes a mechanism for rapid and localized recovery in case of link failure. Colloquially speaking, if a device fails in its attempt to forward a packet to its intended next-hop, then DFF suggests a heuristics for “trying another of that devices’ neighbors”, while keeping track of (and preventing) packet loops.
LOADng is extended to support DFF by adding neighbor discovery mechanisms using HELLO messages. The simulation results show that the DFF could effectively increase the packet delivery ratio at the cost of increasing end-to-end delay and longer path length. More details can be found at:
Five interoperability test events have been organised to improve the protocol specification and verify that interoperable implementations can be developed based on the document. A detailed interop report can be found at Interoperability Report for the Lightweight On-demand Ad hoc Distance-vector Routing Protocol – Next Generation (LOADng).
- Interop 01: The first LOADng interoperability test event was held at Hitachi YRL in Yokohama, Japan, from October 17th to October 19th, 2011. The interoperability tests were conducted according to the specification in LOADng-00. Ecole Polytechnique, Hitachi YRL and EDF R&D participated in the tests. Three implementations (1 from Ecole Polytechnique and 2 from Hitachi YRL) were tested.
- Interop 02: The second LOADng interoperability test event was held at Fujitsu Laboratories of America (FLA), San Jose, USA, on April 13th, 2012. The interoperability tests were conducted according to the specification in LOADng-03. Ecole Polytechnique and Fujitsu Lab of America (FLA) participated in the tests. Two implementations (1 from Ecole Polytechnique and 1 from Fujitsu FLA) were tested.
- Interop 03: The third LOADng interoperability test event was performed at the Los Angeles Airport Hilton, USA, on June 6th, 2012. The interoperability tests were conducted according to the specification in LOADng-04. Ecole Polytechnique and Fujitsu Lab of America (FLA) participated in the tests. Two implementations: 1 from Ecole Polytechnique and 1 from Fujitsu FLA were tested.
- Interop 04: The fourth LOADng interoperability test event was held at Hyatt Hotel, Vancouver, August 2nd, 2012. The interoperability tests were conducted according to the specification in LOADng-05. Ecole Polytechnique and Fujitsu Lab of America (FLA) participated in the tests. Two implementations (1 from Ecole Polytechnique and 1 from Fujitsu FLA) were tested.
- Interop 05: The fifth LOADng interoperability test even was performed at Sagemcom Rueil-Malmaison, France from August 2nd to August 5th, 2016. Ecole Polytechnique, Sagemcom and Nexans participated in the tests. Two implementations (1 from Sagemcom and 1 from Nexans) were tested. The LOADng protocol tested was based on specification “G3-PLC over Medium Voltage Lines: Implementation Guidelines“.
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