You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations 
Chapter Title:  4) Under Specification of DAO Emission 
1. Abstract of Cited Paper (BIB001):  Routing protocols for sensor networks are often designed with explicit assumptions, serving to simplify design and reduce the necessary energy, processing and communications requirements. Different protocols make different assumptions - and this paper considers those made by the designers of RPL - an IPv6 routing protocol for such networks, developed within the IETF. Specific attention is given to the predominance of bi-directional traffic flows in a large class of sensor networks, and this paper therefore studies the performance of RPL for such flows. As a point of comparison, a different protocol, called LOAD, is also studied. LOAD is derived from AODV and supports more general kinds of traffic flows. The results of this investigation reveal that for scenarios where bi-directional traffic flows are predominant, LOAD provides similar data delivery ratios as RPL, while incurring less overhead and being simultaneously less constrained in the types of topologies supported. 
2. Abstract of Cited Paper (BIB002):  With RPL - the “IPv6 Routing Protocol for Low-power Lossy Networks” - emerging as a Proposed Standard “Request For Comment” (RFC) in the Internet Engineering Task Force (IETF) after a ∼2-year development cycle, this paper presents a critical evaluation of the resulting protocol and its applicability and limits. The paper presents a selection of observations of the protocol characteristics, exposes experiences acquired when producing a prototype implementation of RPL, and presents results obtained from testing this protocol - both in a network simulator, and in real-world experiments on a wireless sensor network testbed. The paper aims at providing a better understanding of possible weaknesses and limits of RPL, notably the possible directions that further protocol developments should explore, in order to address these.

Literature Review:
A key issue in constructing RPL downward routes is that the timing of DAO transmission is not explicitly specified. This under-specification of DAO timing may lead to conflict and inefficient implementations of the protocol, consequently harming its performance BIB001 . For instance, the study in BIB002 has opted to transmit DAO messages periodically every 5 seconds, significantly increasing the control overhead compared to the ContikiRPL implementation  , which transmits DAO messages based on the Trickle timers of DIOs. A conservative timing approach may lead to DAOs not being transmitted before old routes expire, affecting negatively the data-plane reliability BIB003 . Hence, an implementation that does not guarantee receipt of all DAOs from intermediate routers along a path would render the root unable to calculate the source route for that destination BIB003 . This is because the accurate calculation of a source route relies on all route segments advertised in the DAOs of its ancestors, up to the DAODAG root. Here, the root would again have no option but to drop all packets for the affected unreachable destination BIB003 .