sensors Review A Comprehensive Overview of TCP Congestion Control in 5G Networks: Research Challenges and Future Perspectives Josip Lorincz 1,* , Zvonimir Klarin 2 and Julije Ožegovi´c 1 1 Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture (FESB), University of Split, 21 000 Split, Croatia; [email protected] 2 Polytechnic of Sibenik, Trg Andrije Hebranga 11, 22 000 Sibenik, Croatia; [email protected] * Correspondence: [email protected] Abstract: In today’s data networks, the main protocol used to ensure reliable communications is the transmission control protocol (TCP). The TCP performance is largely determined by the used congestion control (CC) algorithm. TCP CC algorithms have evolved over the past three decades and a large number of CC algorithm variations have been developed to accommodate various network environments. The fifth-generation (5G) mobile network presents a new challenge for the implemen- tation of the TCP CC mechanism, since networks will operate in environments with huge user device density and vast traffic flows. In contrast to the pre-5G networks that operate in the sub-6 GHz bands, the implementation of TCP CC algorithms in 5G mmWave communications will be further compro- mised with high variations in channel quality and susceptibility to blockages due to high penetration losses and atmospheric absorptions. These challenges will be particularly present in environments such as sensor networks and Internet of Things (IoT) applications. To alleviate these challenges, this paper provides an overview of the most popular single-flow and multy-flow TCP CC algorithms used in pre-5G networks. The related work on the previous examinations of TCP CC algorithm Citation: Lorincz, J.; Klarin, Z.; performance in 5G networks is further presented. A possible implementation of TCP CC algorithms Ožegovi´c,J. A Comprehensive is thoroughly analysed with respect to the specificities of 5G networks, such as the usage of high Overview of TCP Congestion Control frequencies in the mmWave spectrum, the frequent horizontal and vertical handovers, the imple- in 5G Networks: Research Challenges mentation of the 5G core network, the usage of beamforming and data buffering, the exploitation and Future Perspectives. Sensors 2021, of edge computing, and the constantly transmitted always-on signals. Moreover, the capabilities of 21, 4510. https://doi.org/10.3390/ machine learning technique implementations for the improvement of TCPs CC performance have s21134510 been presented last, with a discussion on future research opportunities that can contribute to the Academic Editor: Gianluca Reali improvement of TCP CC implementation in 5G networks. This survey paper can serve as the basis for the development of novel solutions that will ensure the reliable implementation of TCP CC in Received: 31 May 2021 different usage scenarios of 5G networks. Accepted: 28 June 2021 Published: 30 June 2021 Keywords: TCP; congestion control; 5G; mmWave; network; mobile; algorithms; communica- tions; wireless Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction For every new generation of mobile network, the demand for new services and use cases increases. Today’s trends of ubiquitous computing tend to incorporate technology into every device, changing the way that we perform our daily activities and do business. Copyright: © 2021 by the authors. Accommodating these trends imposes implementation and operation challenges for mobile Licensee MDPI, Basel, Switzerland. network operators. To satisfy future needs, the International Telecommunications Union– This article is an open access article Radiocommunications Sector (ITU-R) has defined three main technology advancements distributed under the terms and that fifth-generation (5G) mobile networks should support (Figure1)[ 1]: ultra-reliable and conditions of the Creative Commons low latency communications (URLLC), enhanced mobile broadband (eMBB), and massive Attribution (CC BY) license (https:// machine-type communications (mMTC) in sensor networks. creativecommons.org/licenses/by/ 4.0/). Sensors 2021, 21, 4510. https://doi.org/10.3390/s21134510 https://www.mdpi.com/journal/sensors SensorsSensors2021 2021, 21,, 451021, x FOR PEER REVIEW 2 of 412 of 41 FigureFigure 1. 1.Three Three mainmain use cases of of 5G 5G mobile mobile networks. networks. TheThe usage usage scenariosscenarios characterisedcharacterised as eMBB eMBB aim aim to to provide provide high high data data rates, rates, high high traf- traffic capacity,fic capacity, and and high high seamless seamless mobility mobility for forboth both hotspots and and wide-area wide-area coverage coverage [1]. [ 1As]. As 5G5G mobile mobile technologytechnology is implemented in in phases, phases, its its full full potential potential being being reached reached in the in the upcoming years will enable eMBB use cases to benefit the majority of the population in upcoming years will enable eMBB use cases to benefit the majority of the population in this early phase of 5G mobile network development (Figure 1). The reason for this is that this early phase of 5G mobile network development (Figure1). The reason for this is that the initial implementation of 5G networks is aligned with the consumer market needs, the initial implementation of 5G networks is aligned with the consumer market needs, supporting a rising number of smartphone subscriptions accompanied by an increasing supporting a rising number of smartphone subscriptions accompanied by an increasing mobile data growth rate [2]. In Ref. [3], the minimum requirements for the peak data rate mobilein the dataeMBB growth usage scenario rate [2]. Inare Ref. defined [3], theas 20 minimum Gbit/s for requirements downlink, 10 for Gbit/s the peakfor uplink, data rate inand the 4 eMBB ms for usageuser plane scenario latency. are These defined performance as 20 Gbit/s targets for are downlink, assumed 10for Gbit/s scenarios for with uplink, anda single 4 ms user for usertransferring plane latency.small internet These protocol performance (IP) packets targets for are both assumed the downlink for scenarios and withuplink. a single user transferring small internet protocol (IP) packets for both the downlink and uplink.The URLLC use case presents applications that require ultra-low latencies, reliable communicationThe URLLC and use high case availability presents applications (Figure 1). High that reliability require ultra-low and low latency latencies, are reliablecru- communicationcial requirements and for high emerging availability mission-critical (Figure1). applications High reliability including and low telesurgery, latency are intelli- crucial requirementsgent transportation for emerging and industrial mission-critical automation applications [4]. In URLLC including usage telesurgery,scenarios, the intelligent mini- transportationmum requirement and for industrial user plane automation latency for [4 small]. In URLLCIP packets usage is 1 ms scenarios, for both the downlink minimum requirementand uplink transmission for user plane [3]. latency for small IP packets is 1 ms for both downlink and uplinkMachine-to-machine transmission [3]. communication (M2M) represents machine-centric communi- cationMachine-to-machine between different devices communication and sensors (M2M) without represents human interaction machine-centric (Figurecommunica- 1). With tionthe betweenincreasing different number devices of devices and and sensors sensors without offering human M2M interactioncommunication, (Figure mMTC1). With use the increasingcases will numberbecome more of devices present and in the sensors near offeringfuture. The M2M main communication, driver of increasing mMTC machine- use cases willtype become communication more present is the in advent the near of future.sensor networks The main as driver part of of the increasing Internet machine-typeof Things communication(IoT) paradigm. isThis the is advent where a of very sensor large networks number of as devices part of and the sensors Internet will of communi- Things (IoT) paradigm.cate and exchange This is where information. a very large These number devices of and devices sensors and are sensors characterised will communicate as low cost and exchangeand low power information. with a very These long devices battery and life. sensorsThese devices are characterised and sensors are as lowusually cost trans- and low powermitting with a small a very amount long of battery data that life. is These not sensitive devices to and delays sensors [1]. are usually transmitting a small amountPrevious ofgenerations data that isof notcellular sensitive networks to delays e.g., 3rd [1]. (3G) and 4th (4G), have used fre- quency bands under 6 GHz. As these frequencies are becoming increasingly saturated and Previous generations of cellular networks e.g., 3rd (3G) and 4th (4G), have used as there are limitations that mean that they cannot fulfil the new increasing demands, the frequency bands under 6 GHz. As these frequencies are becoming increasingly saturated need for higher frequency bands above 6 GHz has arisen. Accordingly, 5G mobile net- and as there are limitations that mean that they cannot fulfil the new increasing demands, works are designed to operate on a variety of frequency bands, including the sub-6 GHz the need for higher frequency bands above 6 GHz has arisen. Accordingly, 5G mobile networks are designed to operate on a variety of frequency bands, including