5G TCP and MP-TCP in 5G mmWave Networks Future 5G networks will likely include mmWave radio access communication links, because of their potential multi-gigabit-per-second capacity. However, these frequencies are characterized by highly dynamic channel conditions, which lead to wide fluctuations in the received signal quality. This article explains how the end-to-end user experience in mobile mmWave networks could be affected by a suboptimal interaction between the most widely used transport protocol, TCP, and mmWave links. It also provides insights on the throughput-latency tradeoff when Multipath TCP (MP-TCP) is used judiciously across various links, such as Long-Term Evolution (LTE) and mmWave. Michele Polese he next generation of mobile net- above 10 gigahertz (GHz), indeed, there University of Padova, Italy works (5G) will be standardized is a high availability of contiguous T before 2020, to address mobile spectrum that can be allocated to cel- Rittwik Jana Internet traffic’s growth. According to lular networks. However, mmWaves AT&T Labs–Research the Next-Generation Mobile Network present numerous challenges and (NGMN) alliance, 5G networks shall issues that researchers must address Michele Zorzi provide the following1: a user bitrate of to make this technology market-ready. University of Padova, Italy at least 50 megabits per second (Mbit/s) In fact, these frequencies suffer from at cell edges, with gigabits per sec- high isotropic path loss and blockage ond (Gbit/s) peaks in the most favor- by most solid materials — for example, able conditions; ultra-low end-to-end buildings, cars, and even the human latency (possibly below 10 ms); high body2 — which could result in service reliability and availability of commu- unavailability (that is, an outage). The nications; and support for low-power high antenna gain provided by using massive machine-type communica- multiple antenna techniques, such as tions (MTC). beamforming and massive multiple MmWave communications will play input, multiple output (MIMO), can a major role in meeting the throughput make up for the path loss; but how target of 5G networks. At frequencies to provide a reliable service in the 12 Published by the IEEE Computer Society 1089-7801/17/$33.00 © 2017 IEEE IEEE INTERNET COMPUTING TCP and MP-TCP in 5G mmWave Networks presence of frequent blockages is still an open control algorithms has had several evolutions: question. In particular, a sudden transition the Request for Comments (RFC) 56814 describes of a mmWave link from a line-of-sight (LOS) the most recent one, and there are 13 TCP vari- to a non-line-of-sight (NLOS) channel state ants currently implemented in the Linux ker- generates wide fluctuations in the signal-to- nel.5 In particular, the latest versions of the interference-plus-noise ratio (SINR; in the order major operating systems use TCP CUBIC as the of 30 dB, according to Sundeep Rangan and default.6 This TCP version was designed in order colleagues2), and therefore the capacity offered to react more promptly to packet losses, and to changes drastically, thereby compromising the restore the connection to the highest available end-to-end user experience. rate in a shorter time than legacy designs based These extreme propagation conditions on TCP New Reno. In the latter, after a packet demand a new design of the physical and loss, the sender decreases its congestion win- medium access control (MAC) layers, but also dow by half, and then increases it by one packet have an impact on the interplay with the in each round-trip time (RTT), thus requiring a higher layers of the protocol stack. The most very long time to fill a high capacity link. With widely used reliable data transport protocol TCP CUBIC, instead, the growth is independent is TCP, which however was designed in a dif- of the RTT, which makes it possible to reach a ferent historical context, and according to higher throughput more quickly than in New different needs and constraints. TCP considers Reno, while behaving fairly toward flows using packet losses as an implicit notification of con- other versions of TCP.6 gestion on the link, and therefore reduces the Another trend in modern cellular network- sending rate trying to relieve it. However, the ing is to exploit the presence of multiple net- lossy nature of mmWave links might trigger work interfaces in mobile devices. A typical the TCP congestion control mechanisms even if example is the usage of MP-TCP for vertical and there is no actual congestion. This might yield a seamless handovers between third- and fourth- suboptimal end-to-end performance and waste generation (3G/4G) cellular networks and Wi-Fi the great potential of mmWave links. hotspots.7 MP-TCP is an extension of TCP that With this in mind, here we provide an over- enables multipath transport — that is, the appli- view of the impact of mmWave communications cation communicates with a traditional TCP on the performance of TCP, outlining which are socket, which transparently handles multiple the main novelties that the usage of a mmWave subflows on different interfaces, such as Wi-Fi, radio access network (RAN) introduces in end- the cellular network, and Ethernet. MP-TCP is to-end connections at the transport layer. We currently under discussion in the IETF,8,9 and also consider how adopting the recent Multipath is designed around three main goals.10 First, TCP (MP-TCP) option can improve the perfor- it should improve the throughput, in the sense mance of TCP on these kinds of links. Finally, that it should perform at least as well as a single we suggest possible future research directions. path TCP (SP-TCP) flow on the best path avail- A more technical discussion with additional able. Second, it should not use more resources results is provided elsewhere.3 than standard TCP flows. Finally, it should steer more packets towards less congested paths. Recent Advances in TCP The core of MP-TCP is its congestion control TCP was designed in the 1980s as a connection- algorithm, that manages the congestion win- oriented and reliable protocol that provides dow of each different subflow in a coupled or end-to-end connectivity over multiple hops and uncoupled manner. With an uncoupled conges- congestion control (CC). Reliability is enabled tion control, each subflow is independent, thus by a retransmission mechanism, based on the the congestion window is updated separately in acknowledgments received by the TCP transmit- each path. With a coupled approach, instead, ter. Congestion control is implemented by dif- the congestion window of the different subflows ferent algorithms that increase and/or decrease is increased and decreased in a coordinated the maximum amount of unacknowledged data manner, considering the congestion of all the that the sender is allowed to transmit (the con- available subflows. By coupling the different gestion window), reacting to network events subflows, Costin Raiciu and colleagues10 claim such as packet losses. The original congestion that it is possible to reach the aforementioned SEPTEMBER/OCTOBER 2017 13 5G networks with mmWave links, showing how 50 mmWave 28 GHz state-of-the-art transport protocols interact LTE 2.1 GHz with this technology. 40 The mmWave Channel 30 Because TCP is the most widely used trans- port protocol, it is important to understand the (dB) 20 R interactions that exist between mobile networks SIN (for example, wireless channels) and the TCP 10 performance. In wireless networks, the loss of a packet is not caused necessarily by congestion, 0 but instead might be due to a sudden (and pos- sibly only temporary) drop in signal quality. In 13,14 Ϫ10 related work, the authors study the behavior 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time (s) of TCP in relation to a complex mobile network such as LTE, showing the following: first, as the Figure 1. The ns-3 simulated signal-to-interference-plus-noise ratio distance between the user equipment (UE) and (SINR) for a mmWave and Long-Term Evolution (LTE) link, with the evolved node base (eNB) increases, the TCP the user equipment moving at 2 meters per second (m/s) from throughput degrades; and second, how TCP is (45, 0) to (55, 0), with the evolved node base at coordinates affected by network events such as handovers. (75, 50). The traces are generated using the channel model for the MmWave networks are expected to reach mmWave frequencies (described elsewhere2,15) and the ns-3 LTE an order of magnitude higher throughput than channel model. current systems, thanks to the larger bandwidth available, but present more troublesome prop- agation conditions. As an example, Figure 1 shows a comparison between the time evolution MP-TCP design goals. The first coupled con- of the SINRs of a mmWave link at 28 GHz and gestion control (CC) that they proposed,10 how- an LTE link at 2.1 GHz for a UE that moves at ever, received criticism,11,12 because it transmits 2 meters per second (m/s) at an average distance too much traffic on congested paths and is of 75 meters from the eNB and switches from a unfriendly with respect to SP-TCP. Ramin LOS to a NLOS condition. The main differences Khalili and colleagues11 presented the Oppor- between the mmWave and the LTE channels are tunistic Linked Increases Algorithm (OLIA), as follows: which is designed to overcome these two issues. However, according to Qiuyu Peng and col- • The LOS to NLOS path loss transitions are leagues,12 OLIA presents unresponsiveness deeper for mmWave. Below 6-GHz frequen- problems with respect to congestion changes in cies, these are of the order of 10–15 dB, the subflows.