5G cellular communication, especially with its hugely available bandwidth provided by millimeter-wave, is a promising technology to fulfill the coming high demand for vast data rates. These networks can support new use cases such as Vehicle to Vehicle and augmented reality due to its novel features such as network slicing along with the mmWave multigigabit-per-second data rate. Nevertheless, 5G cellular networks suffer from some shortcomings, especially in high frequencies because of the intermittent nature of channels when the frequency rises. Non-line of sight state, is one of the significant issues that the new generation encounters. This drawback is because of the intense susceptibility of higher frequencies to blockage caused by obstacles and misalignment. This unique characteristic can impair the performance of the reliable transport layer widely deployed protocol, TCP, in attaining high throughput and low latency throughout a fair network. As a result, the protocol needs to adjust the congestion window size based on the current situation of the network. However, TCP is not able to adjust its congestion window efficiently, and it leads to throughput degradation of the protocol. This paper presents a comprehensive analysis of reliable end-to-end communications in 5G networks. It provides the analysis of the effects of TCP in 5G mmWave networks, the discussion of TCP mechanisms and parameters involved in the performance over 5G networks, and a survey of current challenges, solutions, and proposals. Finally, a feasibility analysis proposal of machine learning-based approaches to improve reliable end-to-end communications in 5G networks is presented. INDEX TERMS 5G, end-to-end reliability, mmWave, TCP ANNA CALVERAS AUGÉ was born in Barcelona, Spain, in 1969. She obtained a Ph.D. in Telecommunications Engineering from the Universitat Politècnica de Catalunya, Spain, in 2000. She is an associate professor at the mentioned University, at the Computer Networks Department, in the Wireless Networks Group (WNG). Her research interests and expertise areas comprise the design, evaluation, and optimization of communications protocols and architectures for cellular, wireless multihop networks, ad-hoc networks, wireless sensor networks, the Internet of Things, and application domains such as smart cities, building automation, satellite and emergency environments. She has been involved in several National and International research or technology transfer projects, and she has published in International and National conferences and journals.

By deploying the millimeter-wave wide spectrum in 5G networks, the new generation is capable of providing high data rates with low latencies. However, these frequencies have intermittent characteristics as their downside, which acts as a hurdle on the way of attaining high performances. This disadvantage can lower signals' penetration power in reaching far distances or passing materials such as vehicles, walls, and even human bodies. As a result, having a reliable end-to-end connection throughout 5G millimeter-wave networks can be challenging because this burden is on the transport layer mostly exploited protocol, TCP, which is unable to perform sufficiently due to the fluctuation of the high-frequency channels. This paper aims to analyze TCP's behavior in one of the 3GPP's well-known scenarios called urban deployment. The detailed investigation of TCP over 5G millimeter-wave when used in a city and the impact of different parameters such as remote servers, RLC buffer size, different congestion control algorithms, and maximum segment size are discussed thoroughly throughout the paper. The results revealed that TCP could benefit from the edge server deployment due to the shorter control loop, and increasing maximum segment size can also enhance this superiority. Moreover, individual TCP variants react to various RLC buffer sizes differently. However, in general, increased throughput can be attained by deploying larger buffers at the cost of latency. INDEX TERMS 5G, end-to-end reliability, mmWave, TCP, urban deployment REZA POORZARE received his B.S. and M.S. in computer engineering from Azad University of Iran in 2010 and 2014, respectively. He is currently pursuing his Ph.D. at Universitat Politècnica de Catalunya in network engineering. His research interests include 5G, mmWave, TCP, wireless mobile networks, and Artificial Intelligence. ANNA CALVERAS AUGÉ was born in Barcelona, Spain, in 1969. She obtained a Ph.D. in Telecommunications Engineering from the Universitat Politècnica de Catalunya, Spain, in 2000. She is an associate professor at the mentioned University, at the Computer Networks Department, in the Wireless Networks Group (WNG). Her research interests and expertise areas comprise the design, evaluation, and optimization of communications protocols and architectures for cellular, wireless multihop networks, ad-hoc networks, wireless sensor networks, the Internet of Things, and application domains such as smart cities, building automation, satellite and emergency environments. She has been involved in several National and International research or technology transfer projects, and she has published in International and National conferences and journals.

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5G era opens a new horizon toward communication with new features and capabilities. The new mobile generation comes up with a multi-gigabit per second data rate along with its huge available bandwidths provided by millimeter-wave frequency bands. Dispensing a fully connected world throughout low latency is the supreme aim for 5G networks. However, attaining high-speed, reliable communication is challenging in the new generation, especially in scenarios with numerous obstacles such as the urban one. As the frequency rises, the signal's penetration power declines, which can mislead TCP in adjusting the sending rate because TCP cannot distinguish that a packet drop in a network is due to congestion or other shortcomings of the network such as blockage or random packet drops. This paper proposes a new TCP based on Fuzzy logic, which strives to prevent performance reduction in urban deployments. The Fuzzy rules are implemented in the congestion avoidance phase of the new protocol to adjust the sending rate intelligently and prevent blockage impacts. The ultimate aim of the protocol is to control the sending rate based on the current situation of the network so it can attain the highest possible performance. Moreover, it tries to reach its goal through low latency and keep the average sending rate as small as possible to restrain the buffer exhaustion. The extensive conducted simulations showed that the newly proposed protocol could attain higher performance compared to BBR, HighSpeed, Cubic, and NewReno in terms of throughput, RTT, and sending rate adjustment in the urban scenario.

The demand for high bandwidth on the Internet is growing drastically, and one of the solutions for tackling this problem is using optical networks. Burst switching is one of the techniques that can be used in optical networks to handle high traffic. Aside from many advantages that this technique has, it suffers from a big flaw called burst contention. Optical Burst Switching (OBS) is a switching technique without any buffering system. As a result, when two bursts are trying to reserve one resource, one of them drops. This drawback can have a significant impact on the performance of some protocols like TCP because they have not been designed to perform in a network without any queuing system and cannot distinguish a drop is because of the congestion or contention. In this paper, a new algorithm called AVGR (Average of RTTs) is proposed based on some mathematical equations to prevent the degradation of TCP. It tries to calculate averages for some RTTs in three different periods. Then base on the obtained results, the congestion control mechanism will be modified. The primary goal of the algorithm is to determine the current status of the network and make proper decisions based on it.