A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> C. Unique Routing Challenges in LLNs <s> This paper describes the concept of sensor networks which has been made viable by the convergence of micro-electro-mechanical systems technology, wireless communications and digital electronics. First, the sensing tasks and the potential sensor networks applications are explored, and a review of factors influencing the design of sensor networks is provided. Then, the communication architecture for sensor networks is outlined, and the algorithms and protocols developed for each layer in the literature are explored. Open research issues for the realization of sensor networks are also discussed. <s> BIB001 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> C. Unique Routing Challenges in LLNs <s> We present Trickle, an algorithm for propagating and maintaining code updates in wireless sensor networks. Borrowing techniques from the epidemic/gossip, scalable multicast, and wireless broadcast literature, Trickle uses a "polite gossip" policy, where motes periodically broadcast a code summary to local neighbors but stay quiet if they have recently heard a summary identical to theirs. When a mote hears an older summary than its own, it broadcasts an update. Instead of flooding a network with packets, the algorithm controls the send rate so each mote hears a small trickle of packets, just enough to stay up to date. We show that with this simple mechanism, Trickle can scale to thousand-fold changes in network density, propagate new code in the order of seconds, and impose a maintenance cost on the order of a few sends an hour. <s> BIB002 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> C. Unique Routing Challenges in LLNs <s> This document presents home control and automation application ::: specific requirements for Routing Over Low power and Lossy networks ::: (ROLL). In the near future many homes will contain high numbers of ::: wireless devices for a wide set of purposes. Examples include ::: actuators (relay, light dimmer, heating valve), sensors (wall switch, ::: water leak, blood pressure) and advanced controllers (RF-based AV ::: remote control, Central server for light and heat control). Because ::: such devices only cover a limited radio range, routing is often ::: required. The aim of this document is to specify the routing ::: requirements for networks comprising such constrained devices in a ::: home control and automation environment. <s> BIB003 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> C. Unique Routing Challenges in LLNs <s> This document discusses the applicability of the Routing Protocol for ::: Low-Power and Lossy Networks (RPL) in Advanced Metering Infrastructure ::: (AMI) networks. <s> BIB004
The design of efficient routing protocols for LLNs is driven by the unique characteristics of such networks. The limited memory and processing resources, low data rates and limited power supply in the majority of devices along with the lossy nature of interconnects (links), all need to be addressed. In the following, we shed light on some of the routing process design issues arising in the context of LLNs. Diversity of Applications: Several applications are envisaged to run under the umbrella of LLNs including home/building automation, industrial applications, environmental monitoring, military applications, etc. These diverse applications exhibit characteristics BIB001 [53] and, consequently, different requirements in terms of power consumption, convergence time, traffic overhead, reliability, latency or other performance metrics. Hence, a big challenge for a LLN routing protocol is to accommodate all of these diverse and conflicting requirements within the application's resource budget BIB002 . Communication Patterns: The dominant communication pattern in LLN applications is the Multipoint-to-Point (MP2P)  , in which data is gathered by a group of sensors and reported to a common destination called the LBR or the sink. Other communication patterns also exist, including the Point-toMultiPoint (P2MP), where the sink sends data to the associated sensor nodes and the Point-to-Point (P2P) in which a sensor node communicates with one other in the network BIB003  . This diversity in communication patterns represents another challenge when designing LLNs routing protocols. Reporting Model: The data communication models in LLNs vary widely, but are roughly classified into three categories, query-based, event-based, and time-based BIB001 BIB002 . In the querybased model, data is only reported upon the receipt of an explicit query. In the time-based model, sensing devices report their data of interest periodically at pre-specified time. In the event-based model, sensing nodes only report their readings upon detecting abrupt and significant changes in the value of data of interest. Hybrid models combining two or more of these are also encountered BIB002 . The efficiency of routing protocols in terms of route stability and power consumption is highly sensitive to the reporting model. Scalability: It is envisioned that LLNs will operate in deployments of different densities, ranging from a few neighbors per node to hundreds BIB003 - BIB004 . A protocol should thus be able to handle all cases within the viable range and its parameters should be dynamically tuned according to what it encounters in practice BIB002 . In other words, the scalability is a design issue that should be satisfied by an LLN protocol.