Chapter 1: Introduction¶
The Internet is considered an engineering success with few peers, and rightfully so. It has scaled to connect billions of devices, supports every imagined communications application, and accommodates transmission rates ranging from tens of bits-per-day to hundreds of gigabits-per-second. But at its core is a thorny technical challenge that has drawn widespread attention for the last 30-plus years, from both practitioners trying to make the Internet perform better and theoreticians wanting to understand its mathematical underpinnings: how the Internet’s resources are best allocated to all the competing interests trying to use it.
Resource allocation is a hard problem in any computer system, but especially so for a system as complex as the Internet. The problem was not top-of-mind when the Internet’s TCP/IP protocol stack was first deployed in the early 1980s. By the end of the decade, however, with the Internet gaining serious use in universities (but predating the World Wide Web’s invention by several years), the network began to experience a phenomenon known as congestion collapse . A solution—congestion control—was developed and deployed in the late 1980s and the immediate crisis was addressed. The Internet community has been studying and refining its approach to congestion control ever since. This book is about that journey.
The most famous early efforts to manage congestion were undertaken by two researchers, Van Jacobson and Mike Karels. The resulting paper, Congestion Avoidance and Control, published in 1988, is one of the most cited papers in networking of all time. There are good reasons for that. One is that congestion collapse really did threaten the nascent Internet, and the work undertaken to address it was foundational to the Internet’s ultimate success. Without that work it’s unlikely we’d have the global Internet we have today.
Another reason for the citation impact of this work is that congestion control has been an amazingly fruitful area of research for over three decades. Congestion control, and resource allocation more broadly, are wide open design spaces with plenty of room for innovation. Decades of research and implementation have built on the early foundations, and it seems fair to assume that new approaches or refinements to the existing approaches will continue to appear for as long as the Internet exists.
In this book, we explore the design space for congestion control in the Internet and present a description of the major approaches to managing or avoiding congestion that have been developed over the last three decades.
V. Jacobson. Congestion Avoidance and Control. ACM SIGCOMM ‘88 Symposium, August 1988.
1.1 What is Congestion?¶
Anyone who has driven on a highway at rush hour has experienced congestion. There is a limited resource—the space on the highway—and a set of cars, trucks, etc. that compete for that resource. As rush hour gets underway, more traffic arrives but the road keeps working as intended, just with more vehicles on it. But there comes a point where the number of vehicles becomes so large that everyone has to slow down (because there is no longer enough space for everyone to keep a safe distance at the speed limit) at which point the road actually becomes less effective at moving vehicles. So, just at the point when you would be wanting more capacity, there is actually less capacity to move traffic, as illustrated in Figure 1. This is the essence of congestion collapse, when congestion is so bad that the system starts to perform significantly worse than it did without congestion. The mechanism of congestion collapse is quite a bit different for packet networks than for highways, but it is equally problematic 1.
Networking people like making analogies between real-world congestion and network congestion, but it’s important to recognize that analogies are imperfect.
This book focuses on congestion control for packet-switched networks. A fundamental aspect of packet switching is multiplexing, which is the means by which a system resource—such as a link or a queue in a router—is shared among multiple users or applications. In the case of the Internet, packet networks are statistically multiplexed, which means that, as packets show up somewhat randomly, we rely on the statistical properties of those arrivals to ensure that we don’t run out of resources. The existence of congestion collapse shows that sometimes the statistics don’t quite work out as we’d like.
To see how this might work, consider the simple network illustrated in Figure 2, where the three hosts on the left side of the network (senders S1-S3) are sending data to the three hosts on the right (receivers R1-R3) by sharing a switched network that contains only one physical link. (For simplicity, assume that host S1 is sending data to host R1, and so on.) In this situation, three flows of data—corresponding to the three pairs of hosts—are multiplexed onto a single physical link by switch 1 and then demultiplexed back into separate flows by switch 2. Note that we are being intentionally vague about exactly what a “flow of data” corresponds to for now, but we will make this more precise in later chapters.
Statistical multiplexing means that all the hosts in this network send packets whenever it suits them, and if it happens that several packets turn up at the same time at a switch, one of them will be transmitted first while the others are placed into a queue. So both the link and the queue are shared resources, and both are finite. The link can only carry so many bits per second, and the queue can only hold so many packets (or bytes) before it has to start discarding packets. Managing the access to these shared resources, and trying to do so in a way that prevents congestion collapse, is the essence of congestion control. A switch that occasional puts packets in a queue is operating normally. A switch that has large numbers of packets in its queues all or most of the time is congested. We’ll get to the definition of congestion collapse for networks later on, but it starts with congested switches, routers or links.
For a deeper introduction to statistical multiplexing, and why it’s the approach of choice for packet networks, we refer to the following text.
Requirements. Computer Networks: A Systems Approach, 2020.
When a switch builds a queue of packets awaiting transmission, it needs to decide which packet gets sent next. Each switch in a packet-switched network makes this decision independently, on a packet-by-packet basis. One of the issues that arises is how to make this decision in a fair manner. For example, many switches are designed to service packets on a first-in, first-out (FIFO) basis. Another approach would be to transmit the packets from each of the different flows that are currently sending data through the switch in a round-robin manner. This might be done to ensure that certain flows receive a particular share of the link’s bandwidth or that they never have their packets delayed in the switch for more than a certain length of time. A network that attempts to allocate bandwidth to particular flows is sometimes said to support Quality-of-Service (QoS).
One thing to take away from this discussion is that it is in the nature of packet-switched networks that they will sometimes be congested. The focus of this book is on the large body of work that has been done to mitigate congestion, either by responding to it in effective ways to lessen it, or by preventing it before it occurs.
1.2 Controlling Congestion¶
Resource allocation and congestion control are complex issues that have been the subject of much study ever since the first network was designed. They are still active areas of research. One factor that makes these issues complex is that they are not isolated to a single level of a protocol hierarchy. Resource allocation is partially implemented in the routers, switches, and links inside the network and partially in the transport protocol running on the end hosts. End systems may use signalling protocols to convey their resource requirements to network nodes, which respond with information about resource availability. Application protocols may themselves be designed to mitigate congestion, for example, by changing the resolution of video transmission based on the current network conditions. This is a canonical example of a systems issue: you can’t fully understand congestion without looking at all the places in the system that it touches.
We should clarify our terminology before going any further. By resource allocation, we mean the process by which network elements try to meet the competing demands that applications have for network resources—primarily link bandwidth and buffer space in routers or switches. Of course, it will often not be possible to meet all the demands, meaning that some users or applications may receive fewer network resources than they want. Part of the resource allocation problem is deciding when to say no and to whom.
We use the term congestion control to describe the efforts made by network nodes (including end systems) to prevent or respond to overload conditions. Since congestion is generally bad for everyone, the first order of business is making congestion subside, or preventing it in the first place. This might be achieved simply by persuading a few hosts to stop sending, thus improving the situation for everyone else. However, it is more common for congestion-control mechanisms to have some aspect of fairness—that is, they try to share the pain among all users, rather than causing great pain to a few. Thus, we see that many congestion-control mechanisms have some sort of resource allocation built into them.
It is also important to understand the difference between flow control and congestion control. Flow control involves keeping a fast sender from overrunning a slow receiver. Congestion control, by contrast, is intended to keep a set of senders from sending too much data into the network because of lack of resources at some point. These two concepts are often confused; as we will see, they also share some mechanisms.
Given all the different places and layers where congestion control and resource allocation can be implemented, it is helpful to start with a simple approach, which is pretty much what Jacobson and Karels did (although their solution ended up having quite a few moving parts).
In the early Internet, routers implemented the most basic resource allocation approach possible: FIFO queuing with tail drop. There was no awareness of flows or applications, so they simply accepted packets as they arrived, placed them in a queue whenever the outbound link capacity was less than the arrival rate, served the queue by the FIFO discipline, and dropped arriving packets if they queue was full (“tail-drop”). This is still the most common form of queuing today; we will discuss other approaches to queuing including Active Queue Management in a later chapter.
The reason that congestion collapse occurred in the early Internet is that dropped packets are not just discarded and forgotten. When the end-to-end transport protocol is TCP, as it is for most Internet traffic, a dropped packet will be retransmitted. So as congestion rises, the number of retransmitted packets rises; in other words, the number of packets sent into the network increases even if there is no real increase in the offered load from users and applications. More packets lead to more drops leading to more retransmissions and so on. You can see how this leads to collapse.
A useful term in this context is goodput, which is distinguished from throughput in the sense that only packets doing useful work are counted towards goodput. So, for example, if a link is running at 100% utilization, but 60% of the packets on that link are retransmitted due to earlier losses, you could say the goodput was only 40%.
The key insight of early researchers on congestion control was that it was possible and necessary for TCP to do something other than blindly retransmit lost packets during times of congestion. TCP would have to detect the congestion—which it can do, for example, by noticing the loss of packets—and then respond to the congestion by reducing the amount of traffic sent into the network. This interaction between the end-to-end protocol and the network during times of congestion formed the basis for much of today’s congestion control and avoidance approaches. We’ll get into the specifics of how these approaches work in subsequent chapters.
1.3 Theoretical Underpinnings¶
There has been a lot of important theoretical work done to understand congestion. At the core of congestion is queuing, and there is a huge body of theory behind queuing, much of which extends into other physical realms such as supermarket checkouts and road congestion. The standard reference on queuing for packet networks was written by one of the early pioneers of the ARPANET, Leonard Kleinrock.
L. Kleinrock. Queueing Systems, Volume 2.
As packet networks became more widespread in the 1980s, there was a great deal of interest in how traffic behaved, with a growing realization that it might be more complex than had first been thought. One of the most popular models for data traffic was the Poisson model, which had worked well for various systems like call arrivals in the telephone network and people arriving at a queue in a supermarket. But the more that people studied the Internet and other packet networks, the worse the Poisson model started to look. There are a number of seminal papers that make the case for more complex models, of which the following are two.
V. Paxson and S. Floyd. Wide-Area Traffic: The Failure of Poisson Modeling. IEEE/ACM Transactions on Networking, June 1995.
W. Leland et al, On the self-similar nature of Ethernet traffic. ACM SIGCOMM ‘93 Symposium, August 1993.
These papers and others contributed to the consensus that Internet traffic is much more “bursty”—packets arrive in clumps—than had been assumed by early models. Furthermore, this burstiness displays self-similarity—a property of fractals, whereby when you zoom in, you keep seeing similar complexity at finer resolutions. For Internet traffic, this means that at any time scale, from microseconds to hours, you will see similar sorts of patterns.
This research had a number of practical consequences, such as the realization that packet queues might get to be very long indeed, and thus routers and switches should have reasonably large packet buffers. (Correctly sizing those buffers became its own research topic.) Link utilizations could not be reliably kept close to 100% all the time, because you had to allow room for unpredictable bursts.
Two topics of particular importance when thinking about congestion avoidance are fairness and stability. When the network is congested, it’s going to be necessary for some users or flows to send less. It is clearly worth asking: which flows should send less? Should all flows share the pain equally? And what happens if some flows pay more attention to congestion signals than others? These questions are at the heart of the fairness issue. Jain’s fairness index is one of the widely accepted ways to measure how fair a network is. We dig into this topic in Chapter 3.
Stability is a critical property for any sort of control system, which is what congestion control is. Congestion is detected, some action is taken to reduce the total amount of traffic, causing congestion to ease, at which point it would seem reasonable to start sending more traffic again, leading back to more congestion. You can imagine that this sort of oscillation between congested and uncongested states could go on forever, and would be quite detrimental if the network is swinging from underutilized to collapsing. We really want it to fine an equilibrium where the network is busy but not so much so that congestion collapse occurs. Finding these stable control loops has been one of the key challenges for congestion control system designers over the decades. The quest for stability features heavily in the early work of Jacobson and Karels and stability remains a requirement that subsequent approaches have to meet.
Once the initial congestion control algorithms of TCP were implemented and deployed, researchers began to build mathematical models of TCP’s behavior. This enabled the relationship between packet loss rate, round-trip time, and throughput to be established. The foundation was laid in the paper by Mathis and colleagues, but there has been a body of work that is ongoing as the congestion control algorithms evolve. The idea that TCP would converge to a certain throughput given stable conditions of RTT and loss also formed the basis for TCP-friendly rate control (TFRC). TFRC extends TCP-like congestion control to applications that don’t use TCP, based on the idea that they can still share available capacity in a fair way with those that do. We return to this topic in Chapter 7.
M. Mathis, J. Semke, J. Mahdavi, and T. Ott. The Macroscopic Behavior of the TCP Congestion Avoidance Algorithm. SIGCOMM CCR, 27(3), July 1997.
Finally, much of the theoretical work on congestion control has framed the problem as “a distributed algorithm to share network resources among competing sources, where the goal is to choose source rate so as to maximize aggregate source utility subject to capacity constraints.” Formulating a congestion-control mechanism as an algorithm to optimize an objective function is traceable to a paper by Frank Kelly in 1997, and later extended by Sanjeewa Athuraliya and Steven Low to take into account both traffic sources (TCP) and router queuing techniques (AQM).
F. Kelly. Charging and Rate Control for Elastic Traffic. European Transactions on Telecommunications, 8:33–37, 1997.
S. Athuraliya and S. Low, An Empirical Validation of a Duality Model of TCP and Active Queue Management Algorithms. Proceedings of the Winter Simulation Conference, 2001.
This book does not pursue the mathematical formulation outlined in these papers (and the large body of work that followed), but we do find it helpful to recognize that there is an established connection between optimizing a utility function and the pragmatic aspects of the mechanisms described in this book. Congestion control is an area of networking in which theory and practice have been productively linked to explore the solution space and develop robust approaches to the problem.
1.4 Congestion Control Today¶
It sometimes feels like networking protocols have all been nailed down and standardized for decades, but few areas have remained as dynamic as congestion control. While the early work by Jacobson, Karels and others laid the foundation, there has been a long series of innovations that continue today. We cover many of these in detail in subsequent chapters, but you can rest assured that new ideas in congestion control will continue to emerge for years to come.
Sometimes innovations are necessitated by changes in the landscape. For example, as bandwidths increased from megabits to gigabits per second, the amount of data in flight at any instant increased, which raises the stakes for detecting and responding to congestion quickly. High latency links, such as trans-oceanic cables and satellite links added to this problem by raising the round-trip time (RTT). These situations led to such innovations as using delay (and changes to delay) as a congestion signal (first seen in TCP Vegas). Also, with these “fatter pipes”, there is a greater incentive to get the pipe filled quickly; you don’t want to spend 10 RTTs figuring out how quickly you can send data if your message could have been sent in one or two RTTs. This led to efforts to more quickly determine the bottleneck bandwidth, such as XCP, RCP, and Quick-start for TCP.
Wireless networks, which became mainstream long after the early days of TCP, added a new issue to the mix: packet loss was no longer a reliable congestion signal, but could instead be attributed to a noisy radio channel. This led to a range of approaches to either hide the loss from the TCP hosts or to improve the mechanisms by which TCP detects congestion.
Cloud datacenters became another “use case” for congestion-control mechanisms. Unlike the Internet in general, where end-to-end latencies are highly variable, the RTT in a datacenter is both predictable and relatively small (<10ms). And because the network is highly regular in structure (e.g., a leaf-spine fabric) it is well-understood where and under what circumstances congestion is likely to occur. This makes TCP running in a datacenter ripe for a purpose-tuned algorithm rather than having to use the general-purpose mechanism that runs on the global Internet.
New applications have also contributed to the interest in improving congestion control. One salient example is the rise of video streaming as the (currently) dominant source of traffic on the Internet. Again, there were many approaches developed to make video work better under conditions of congestion. One that has enjoyed great success is Dynamic Adaptive Streaming over HTTP (DASH), in which the server delivering the video switches from one quality of encoding to another (and hence from one bit-rate to another) in response to the measured congestion on the path to the receiver. This moves the congestion control loop up to the application layer, or rather, it adds a second control loop on top of the one already provided by TCP.
This quick tour of innovations is hardly exhaustive, and we will see more detail on these and other approaches in the coming chapters. The important thing to understand at this point is that congestion control continues to evolve as the technology landscape and application requirements change.
1.5 Reference Implementation¶
We saw in Section 1.3 that there is a rich body of literature studying the mathematical properties of congestion-control algorithms, yet congestion control remains a highly pragmatic concern. It is estimated that TCP connections carry 85% of the traffic on the Internet, and those connections are anchored in software implementations of TCP running in every imaginable OS (e.g., Linux, Windows, MacOS, iOS, Android). As a practical matter, the very specification of the congestion-control mechanisms we discuss in this book is represented in kernel-level code, typically implemented in C. The theory defines abstract models of this code, but the code specifies the algorithm.
If the implementation is effectively the specification, then which implementation is authoritative; which is the reference implementation? The answer has been the dominant open source implementation of the day. This was originally the Berkeley Software Distribution (BSD) implementation of Unix, and in fact, the initial algorithm proposed by Jacobson and Karels was a noteworthy feature of the Tahoe release of BSD 4.3 in 1988. This connection between BSD Unix and the TCP congestion-control algorithms was so strong that the variants of algorithm became known (named) according to the BSD release: e.g., TCP Tahoe, and later TCP Reno.
S.J. Leffler, M.K. McKusick, M.J. Karels, and J.S Quarterman. The Design and Implementation of the 4.3 BSD Unix Operating. Addison-Wesley. January 1989.
BSD and its descendants continue to this day (notably as FreeBSD), but it was eventually overtaken by Linux, in the early 2000s, as the de facto open source, Unix-based OS. All the variants of TCP congestion control described in this book are available (and can be optionally activated) in the Linux kernel. They have become the reference implementation of those algorithms, which leads us to our final point: The standard for evaluating TCP congestion-control mechanisms is empirical, by running real traffic between Linux-based implementations of TCP senders and receivers. The open question is: What traffic, and over what network?
While useful insights can often be gained by observing the behavior of TCP connections running across the actual Internet, the wide variability (in both time and space) of “the Internet” makes controlled experiments virtually impossible. Instead, the current best-practice is to run a collection of “representative flows” over isolated but “representative network topologies.” There is no established gold standard for either the set of flows or the set of network topologies, so experimental results are never definitive. But the body of evidence collected using this methodology as proven sufficient to advance the state-of-the-art over the years.
For the purposes of this book, we use the experimental methodology described in Chapter 3. We use it to both visualize the behavior of the various algorithms (helping to build intuition) and to highlight problematic scenarios that continue to make congestion control such a challenging and interesting technical problem.