# SDN-Based Routing for Backhauling in Ultra-Dense Networks
## 2 Related Literature  
SDN has been widely investigated for wired networks, where its benefits have been proved both in terms of performance and management. As a consequence, there is an increasing interest in evaluating if also mobile networks can take advantages from SDN [5; 6]. Benefits and challenges of the software-defined-networking in wireless networks are analyzed in several papers that present high-level architectures [7; 8; 9]. In particular, there is a great interest in SDN-based backhauling due to the dominant role that these networks will have in future 5G systems, especially in presence of UDNs and new paradigms such as Cloud-RAN, caching, fog/edge computing [10; 11; 12]. In this area, recent papers focus on specific services provided by an SDN-based backhaul network, such as mobility management [13; 14; 15], multi-tenant network slicing [16; 17], caching [18; 19] and cross-layer coordination [20].  
For what concerns routing schemes, the literature mainly refers to wired and fixed networks. In particular, the shortest-path routing protocols are the most widely used and investigated. Main research efforts have been focused on bandwidth-constrained solutions, where a new flow can be admitted only if there is a route that can support its bandwidth demand. Recently, also bandwidth-delay constrained problems have been considered. Using SDN paradigm, paths between source and destination can be defined with more flexibility, thus trying to overcome limits of the classical routing, and mainly aiming at minimizing the maximum link use of the network. An exhaustive survey of quality-of-service (QoS) routing strategies for SDNs is provided in [21], while a comparison between routing on conventional and software-defined wired networks is presented in [22] in terms of convergence time. Moreover, different SDN-based routing strategies have been recently investigated for wired networks. For example, in [23] a hybrid SDN is presented, combining the flexibility of SDN with the robustness of Open Shortest Path First (OSPF) algorithm. A multipath routing protocol that integrates also load balancing and admission control is proposed in [24], while the network use is maximized in [25] solving a bandwidth-delay constrained problem.  
By contrast, few papers are available on SDN-based routing in mobile network backhauling. In this area, the attention is mainly devoted to wireless backhauling at _mmWave_ frequencies, which is a promising solution for small cells. The focus is on multi-hop transmissions, whose goal is to increase system throughput jointly optimizing resource allocation and capacity-aware routing [26; 27; 28; 29; 30]. Routing in a wireless SDN (i.e., not a backhaul network) is investigated in [31], where a modified shortest-widest path algorithm is proposed. Conversely, actual backhaul networks for UDNs made of heterogeneous links and multi-hop connections are considered in [32; 33]. A distributed anycast routing protocol is proposed in [32] for a conventional network. Differently, [33] considers an SDN-approach that mainly focuses on jointly solving the problems of routing in the backhaul network and user equipment (UE) cell association, following a cross-layer approach.