A Survey on Recent Advances in Transport Layer Protocols
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This paper has been accepted for publication in IEEE Communications Surveys & Tutorials, DOI 10.1109/COMST.2019.2932905. 1 This is a preprint version of the accepted paper. Copyright (c) 2019 IEEE. Personal use of this material is permitted. A Survey on Recent Advances in Transport Layer Protocols Michele Polese, Student Member, IEEE, Federico Chiariotti, Member, IEEE, Elia Bonetto, Filippo Rigotto, Andrea Zanella, Senior Member, IEEE, Michele Zorzi, Fellow, IEEE Abstract—Over the years, the Internet has been enriched with driving, are going to require more from the network and im- new available communication technologies, for both fixed and pose ever more stringent Quality of Service (QoS) constraints. mobile networks and devices, exhibiting an impressive growth in This, along with the increasing heterogeneity of the network, terms of performance, with steadily increasing available data rates. The Internet research community has kept trying to makes the role of transport protocols more important, and, evolve the transport layer protocols to match the capabilities at the same time, more challenging. Indeed, the end-to-end of modern networks, in order to fully reap the benefits of the performance and the Quality of Experience (QoE) of the users new communication technologies. This paper surveys the main largely depend on the interaction among the applications, the novelties related to transport protocols that have been recently transport layer and the underlying network [11]. In particular, proposed, identifying three main research trends: (i) the evolution of congestion control algorithms, to target optimal performance the transport layer, which is responsible for the management in challenging scenarios, possibly with the application of machine of the end-to-end connection over any number of network learning techniques; (ii) the proposal of brand new transport hops, has to adapt and evolve in order to let users fully benefit protocols, alternative to the Transmission Control Protocol (TCP) from the aforementioned innovations. However, a number of and implemented in the user-space; and (iii) the introduction of factors prevent new solutions at the transport layer from being multipath capabilities at the transport layer. widely adopted, and, in recent years, the research community has been forced to cope with these limitations and identify innovative solutions in order to have significant effects on I. INTRODUCTION Internet performance. The communication technologies that provide access and In particular, the deployment of alternative transport pro- backhaul connectivity to the Internet have dramatically tocols, such as the Stream Control Transmission Protocol changed since the 1980s, when the protocols that are part (SCTP) [12], is slowed down by the widespread use of mid- of today’s TCP/IP stack were first introduced [1]–[3]. Traffic dleboxes [13], [14], which often drop packets from protocols generated on mobile devices is expected to exceed desktop which are different from the Transmission Control Protocol and server traffic by 2021 [4]. New communication standards (TCP) and/or the User Datagram Protocol (UDP) [15]. More- are being proposed and launched to market every few years. over, the socket Application Programming Interface (API) Driven by the increase in web and multimedia traffic demand, (offered by the Operating System kernel and supported by mobile and fixed networks are rapidly evolving. 3GPP NR [5], TCP/UDP) is almost universally used [16], thus limiting the [6] will bring ultra-high data rates with low latency to future interfacing options between application and transport layers to 5G devices, and, similarly, the IEEE 802.11 standard will tar- what the API supports. Finally, the most widespread Operating get ultra-dense deployments [7] and multi-gigabit-per-second Systems implement the transport functionalities (i.e., TCP and throughput [8]. Modern devices are capable of connecting to UDP) in the kernel, making the deployment of new solutions heterogeneous networks [9], and fixed networks are using new difficult. These elements define what is called transport layer arXiv:1810.03884v2 [cs.NI] 31 Jul 2019 optical technologies and Software-Defined Networking (SDN) ossification [16], a phenomenon which has pushed developers for unprecedented rates and low latency [10]. and researchers to only use legacy TCP (for reliable traffic The increasing capabilities of the network make new kinds and congestion control), even though it may not be the best of applications possible; the exponential growth of multimedia performing protocol for the desired use case. TCP has indeed or real-time traffic [4] would have been impossible without the some performance issues in specific scenarios, e.g., on wireless recent technological advances. As networks progress towards links with high variability [17], [18], Head of Line (HoL) 5G, new kinds of applications, such as Augmented Reality blocking with web traffic [19], and bufferbloat [20]. 1 (AR) and Virtual Reality (VR) or autonomous cooperative This survey focuses on three directions in transport layer The authors are with the Department of Information Engineering (DEI), research (i.e., new transport protocols, congestion control in- University of Padova, Padova, 35131, Italy. Email:{polesemi, chiariot, bonet- novations and multipath approaches) that have emerged in the toe, rigottof, zanella, zorzi}@dei.unipd.it last fifteen years to solve the aforementioned problems. New This work was partially supported by the program Supporting Talent in Re- search@University of Padua: STARS Grants, through the project "Cognition- congestion protocols have been proposed to target low latency Based Networks: Building the Next Generation of Wireless Communications and full bandwidth utilization [21], [22]. Novel transport Systems Using Learning and Distributed Intelligence," and by the US Army protocols have been discussed by the Internet Engineering Research Office under Grant no. W911NF1910232: "Towards Intelligent Tactical Ad hoc Networks (TITAN)." Task Force (IETF), with technical novelties such as multipath 1A comprehensive list of acronyms is provided at the end of the paper. capabilities, to exploit the multiple interfaces available in 2 modern smartphones, computers and servers, and user space latency, packet loss, buffer state and size, and volatility of implementations, to overcome the ossification that prevents the capacity) can affect the transport layer performance [34]. a widespread adoption of novel algorithms at the transport In order to overcome this problem, more and more complex layer. Therefore, in this survey, we first review the main new congestion control mechanisms have been proposed, going far transport protocols that have been proposed or standardized by beyond the original simple Additive Increase Multiplicative the IETF since 2006: we provide a brief overview on SCTP Decrease (AIMD) principle. We will describe the main design and Datagram Congestion Control Protocol (DCCP) [23], and philosophies in greater detail in Sec. IV. In fact, researchers are then delve into a more recent contribution, i.e., the Quick questioning TCP’s extensive use as a one-size-fits-all solution, UDP Internet Connections (QUIC) [24] protocol. Then, we because all these factors can cause performance issues that are review the research on congestion control. We describe both becoming more and more apparent. Some of these issues are new mechanisms using classic approaches, e.g., Bottleneck fundamental problems of the transport layer abstraction, while Bandwidth and Round-trip propagation time (BBR) and Low others depend on the specific features of the protocol and can Latency (LoLa) for TCP, and some novel proposals for using be avoided by designing the protocol correctly. This section machine learning techniques for congestion control. Finally, provides a short review of some of the most important issues, the third trend we discuss in this survey is related to the while the rest of the paper is dedicated to the discussion of adoption of multipath solutions at the transport layer, mainly the several solutions that have been proposed to address these with Multipath TCP (MPTCP) [25], [26], but also with challenges. multipath extensions for SCTP [27], QUIC [28], [29], and with the Latency-controlled End-to-End Aggregation Protocol A. Bufferbloat (LEAP) [30]. Our goal in this survey is to offer the interested reader a As we will discuss in depth in Sec. IV, congestion control comprehensive point of view on the up-to-date research on mechanisms exploit an abstract view of the underlying network transport protocols, which is lacking in other recent surveys to tune the amount of data to be sent. As shown in [20], that separately focus on ossification [16], multipath transmis- however, this abstraction in some instances fails to provide sions [31], or congestion control schemes for MPTCP [32] or accurate information on the links connecting the two hosts in data centers [33]. and leads to degraded performance. In particular, when large The remainder of the paper is organized as follows. In buffers are deployed before a bottleneck in order to prevent Sec. II we describe the main issues and limitations of trans- packet losses, then loss-based TCP probing mechanisms in- port protocols in modern networks. Then, in Sec. III, we crease the queue occupancy, thus causing a spike in latency. describe three recently proposed transport protocols, focus- Moreover, since the currently implemented versions of TCP ing in particular on QUIC.