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:  C. Metric Composition 
1. Abstract of Cited Paper (BIB001):  RPL is an IPv6 routing protocol for low-power and lossy networks (LLNs) designed to meet the requirements of a wide range of LLN applications including smart grid AMIs, industrial and environmental monitoring, and wireless sensor networks. RPL allows bi-directional end-to-end IPv6 communication on resource constrained LLN devices, leading to the concept of the Internet of Things (IoT) with thousands and millions of devices interconnected through multihop mesh networks. In this paper, we investigate the load balancing and congestion problem of RPL. Specifically, we show that most of packet losses under heavy traffic are due to congestion, and a serious load balancing problem exists in RPL in terms of routing parent selection. To overcome this problem, this paper proposes a simple yet effective queue utilization based RPL (QU-RPL) that significantly improves end-to-end packet delivery performance compared to the standard RPL. QU-RPL is designed for each node to select its parent node considering the queue utilization of its neighbor nodes as well as their hop distances to an LLN border router (LBR). Owing to its load balancing capability, QU-RPL is very effective in lowering the queue losses and increasing the packet delivery ratio. We verify all our findings through experimental measurements on a real testbed of a multihop LLN over IEEE 802.15.4.

Literature Review:
We understand the need for combining more than one metric to achieve more than one application goal. However, a number of factors are to be taken into consideration when designing a composite metric. First, the number of involved metrics should be kept minimal, as a large number of combined metrics may only be counterproductive. This is because that the more number of combined metrics, the larger is the size of DIOs, which increases the risk of fragmentations and routing errors  . Second, for additive composition, the process of assigning weights for combined metrics becomes more difficult as the number of metrics increases. Hence, we advise that lexical composition should be preferred over additive composition. Third, cross-layer routing design, where multiple metrics from different layers are combined, seems to provide more efficient solutions for routing in LLNs BIB001 BIB002 . The cross-layer approach can be utilized to satisfy different requirements such as load-balancing BIB002 and energy-efficiency BIB001 .