End-to-End Congestion Control

Read this tutorial to understand congestion management at different levels: end-to-end, router assisted, and pricing based.

Overview. Principles and practice of unicast congestion control. Theory of linear control algorithms TCP congestion avoidance and control.

1 The problem

Congestion management is a fundamental problem in networking because the way to achieve cost-effective and scalable network designs is by sharing the network infrastructure. Managing shared resources in a careful way is therefore a critical problem.

The first question one may ask at this stage is "What resources are being shared that need to be managed?" To answer this question, let s think about some of the properties of best-effort networks from the previous lectures. Recall that the model at any router is for each packet to be processed and forwarded along one of several possible output links, at a rate determined by the rated bandwidth of that link. In the meantime, more packets, which in general could belong to any other flow, could arrive at the router (recall that this is one of the key consequences of asynchronous multiplexing).

Figure 1 A simple network topology showing how sharing causes resource contention. Many connections between senders and receivers contend for the bottleneck 100 Kbps link.

What happens to these packets? The router tries to accommodate them in its queues and process them, but if there isn t enough space in its queue, some of them may be dropped. Thus, packet queues build up when the link is busy and demand for the link outstrips the available link bandwidth, and packets are generally dropped when the queues get full. Figure 1 shows a simple network with connections between senders Si and receivers Ri via a 100 Kbps bottleneck link.

The above discussion makes it clear that there are two resources that flows contend for in a network

  1. Link bandwidth. The network has to decide how to apportion bandwidth between different flows. Network routers may also decide to prioritize certain types of packets (e.g., latency-sensitive audio or interactive telnet packets) over others (e.g., electronic mail).
  2. Queue space. When the router decides that a packet has to be dropped because it is running out of queue space (also known as buffer space), which packet should it drop? The arriving one? The earliest one? A random one? And when should it decide to drop packets only when the queue is full, or sooner than that? Waiting too long to before dropping packets only serves to increase packet delays, and it may be advantageous to drop occasional packets even when the queue isn t full.

What happens if we don t manage network resources well? or one thing, the available bandwidth might end up being greatly under-utilized even when there is demand for it, causing economic heartburn. Most often, however, network designers end up provisioning for a certain amount of "expected" offered load, and then have to deal with overload, or congestion.

1.1 Understanding congestion

A network link is said to be congested if contention for it causes queues to build up and for packets to start getting dropped. At such a time, demand for link bandwidth (and eventually queue space), outstrips what is available. or this reason, many network resource management problems are also called congestion control or congestion management problems, since the efficient management of congestion implies an efficient management of the above network resources.

Congestion occurs due to overload, and is exacerbated by network heterogeneity. Heterogeneity is a wonderful property of the Internet, which allows 14.4Kbps wireless links to co-exist with and connect to 100 Mbps Ethernets. However, the transition between links of very different speeds implies that sources connected to high-bandwidth links can t simply blast their packets through, since the bottleneck available bandwidth to a destination can be quite a bit smaller. Sources should learn what rates are sustainable and adapt to available bandwidth this should be done dynamically because there are few paths on the Internet whose conditions are unchanging.

There are many examples of how overload causes congestion. or example, one possible way to achieve high throughput in a network might be to have all the sources blast packets as fast as they can, so that bottleneck network links run at close to 100% utilization. While this seems reasonable, a little thought shows that this approach is self-defeating. All it really accomplishes are long packet queues and resultant end-to-end delays, as well as increased packet loss rates, which for a reliable end-to-end transport layer would cause a large number of retransmissions. This is therefore quite the wrong thing to do to obtain good network throughput. This example also demonstrates the fundamental tension between achieving high link utilizations on the one hand (by transmitting data rapidly), and low delays and loss rates on the other (which increase if data is sent too rapidly).

Figure 2 Schematic view of throughput vs. offered load, showing the optimal operating point and congestion collapse region.

This notion of how increasing offered load to achieve high utilization is at odds with packet losses and delays is illustrated in Figure 2. This figure shows a schematic view of throughput as a function of the offered load on the network. Initially, at low levels of offered load, throughput is roughly proportional to offered load, because the network is under-utilized. Then, throughput plateaus at a value equal to the bottleneck link bandwidth because packets start getting queued.

After a while, when offered load is increased even further, the throughput shows a "cliff " effect, starting to decrease and eventually go to zero! Now, all the competing flows are sending data rapidly, but no user is getting any useful work done (low network throughput). The network has just suffered congestion collapse-a situation where observed throughput decreases in response to an increase in offered load. While seemingly impossible, this catastrophic result is quite real and occurs in a number of scenarios for example, the mid-1980s did see such an event, with large portions of the then Internet coming to a standstill.

Congestion collapse is an example of an emergent property of a complex system, one that shows up rather suddenly in certain situations when the network size and load exceeds a certain amount. Congestion control protocols attempt to reduce or eliminate the occurrence of such nasty situations. In particular, the goal for a good network resource management algorithm would be to operate near the left knee of the curve in Figure 2, by modulating the sources to provide the appropriate optimal offered load.

2 Goals and constraints

At first sight, it might seem that network resource management problems are not that hard to solve. A particularly simple approach would be for a centralized network manager to arbitrate and decide on how link bandwidths and queue buffers are apportioned between different flows, and perform hard-edged admission control (we will see what this means when we talk about Internet quality of service or see Chapter 4 of the .033 lecture notes for what this means) when new flows are started. Of course, this "solution" is impractical for networks larger than only a small number of nodes.

A second approach might be to throw some money at the problem and hope it will go away. In particular, we could decide to provision lots of queue space in our network routers, so the worst that happens upon overload is that packets get queued and are rarely discarded. Unfortunately, all this does is to increase the delays incurred by new packets and does not improve the throughput or efficiency of the system at all. This is a little bit like long lines at an amusement park, where people are waiting to go on a popular roller coaster ride adding more and more space for people to wait in line doesn't improve system throughput at all! The real purpose of queues is primarily to absorb bursts of data that arrive from senders, without causing long delays. We need queues, but we should be careful how many packets we queue at each router persistent queueing of packets is not a good thing.

What are the goals of network resource management in large, heterogeneous networks like the Internet? There are several desirable properties that any good solution must possess, which make the problem interesting and challenging

  • Efficiency. We would like our solutions to lead to high network utilization. What this translates to is high end-to-end application throughputs and low end-to-end delays for all the competing flows.
  • Fairness. The next goal is fairness -- we want fair allocations of resources to competing flows. After all, it is possible to achieve high efficiency by starving all flows but one, obviously an undesirable end result.

It turns out that coming up with the right metric for fairness is tricky, and there are many possible definitions. One possible way of measuring fairness is via a fairness index, which is a function of the mean and variance of the throughputs achieved by different streams. If x1, x2,... , xn are the throughputs of n compenting flows, the fairness index f is defined as

f = \frac{(\sum^{n}_{i=1} x_i)^2}{n \sum^{n}_{i=1} x_i^2}

Clearly, 1/n \leq f \leq 1 with a smaller value of f signifying a larger degree of unfairness in allocations.

One common, but by no means the only, notion of fairness is max-min fairness. According to this, the allocation of a resource is fair if

  1. no user receives more than their request, \rho_i.
  2. no other allocations satisfying (1) has higher minimum allocation
  3. condition (2) recursively holds as we remove the minimal user and reduce the total resource accordingly.

This condition reduces to\mu_i = MIN(\mu_{fair}, \rho_i), where \mu_i is the allocation for user i and (\rho_i\) is user i's request. It assumes that all users have equal rights to the resource, although a weighted version can be analogously defined.

We emphasize that in practice achieving a mathematically precise notion of fairness isn t as important as something that works "reasonably well." (This is a point on which reasonable people in the networking field disagree! It has also been subject to extensive and continuing debate.) We will use this notion of fairness when we talk about fair queueing a couple of lectures from now.

  • Distributed operation. or even moderately sized networks, we can't assume the existence of a centralized oracle deciding what different flows should do.
  • Convergence. Because of the inherently distributed nature of the system, each node (router or end-host) only has incomplete knowledge of the traffic in the network, with which it must act. An important goal of a practical algorithm is that it must not display large oscillatory behavior, and must try to converge towards high efficiency and fairness.

3 End-to-end flow Control

A special case of the "congestion" problem is when the receiver is the bottleneck (e.g., because it has a slow processor or an application that s reading data off the transport stack too slowly). We devote this section to a brief discussion of this problem.

Consider an extremely simple network, a sender connected to a receiver over a fast 100 Mbps link via a fast switch. Suppose the sender is a fast 450 MHz PC, while the receiver is a slower 100 MHz machine. Assume that any sent data arrives in an 8-KByte receive buffer for the receiver to process. Clearly, if the sender sent data faster than the receiver could process, its buffer would overflow and packets will be lost. The receiver and sender need to negotiate how much data the sender is allowed to send at any point in time, and the sender must not violate this agreement. This agreement is called flow control, and is the result of negotiation between the end-to-end transport layers at the receiver and sender.

Most protocols implement flow control via windows. Here, every acknowledgment packet (ack) to the sender also tells the sender how much the sender can send at this point without overflowing receiver buffers. The sender can now be reasonably certain that as long as this agreement is not violated, no packets will be dropped because of overflow at the receiver.

Ensuring flow control is relatively easy-just have a simple agreement between sender and receiver on how much each window can be. However, this does not ensure that packets will not be lost anywhere else, because the network could easily be the bottleneck and become congested. or example, this can easily happen for the topology shown in Figure 1, especially when several flows are sharing the 100 Kbps link. To function well, the network needs a congestion control mechanism, which is significantly harder because neither the sender nor the receiver has precise knowledge about where the bottleneck might be or what the bottleneck bandwidth is.

Thus, flow control is an end-to-end protocol to avoid overwhelming the receiver application and is different from congestion control, which uses a combination of techniques to avoid overwhelming the routers and links in the network. low control is usually achieved via a periodic window-based negotiation between sender and receiver. It is also significantly easier to solve than congestion control, as we will soon discover. Networks need both to function well.

4 End-to-end congestion control

End-to-end congestion control protocols address two fundamental questions:

  1. How do senders react to congestion when it occurs, as signified by dropped packets?
  2. How do senders determine the available capacity for a flow at any point in time?

When congestion occurs, all the flows that detect it must reduce their transmission rate. If they don t do so, then the network will remain in an unstable state, with queues continuing to build up. To move toward the optimal operating point of Figure 2, delays and loss rates must decrease, which can only happen if the congested flows react to congestion by decreasing their sending rates. A decrease algorithm determines how this is done by the end-to-end layer.

When packets are successfully being received and acknowledged by the receiver, the sender can deduce that the current rate is sustainable by the network. However, it does not know if the network can sustain still higher rates because available bandwidth varies with time as users start and depart, and so it needs to carefully probe for spare capacity to use. An increase algorithm determines how this is done by the end-to-end transport layer.

Before we can talk about the details of specific decrease and increase algorithms, we need to understand how the end-to-end transport layer at the sender modulates its transmission rate. We study a class of protocols called window-based protocols for doing this, since it is the most common way of doing it today (e.g., TCP uses this technique). Here, the sender maintains a window, also called the congestion window, or cwnd, that it uses to decide how many unacknowledged packets it can have in the network at any time. This window is different from the flow control window that we discussed before, which is negotiated by the sender and receiver to avoid overrunning the receiver s buffers. In contrast, cwnd is dynamically adjusted by the sender to try and track the state of congestion and available bandwidth in the network.

When the sender has packets to transmit, it sends out only as many as the minimum of these two windows permit, thereby ensuring that the number of outstanding, unacknowledged packets does not overrun the receiver nor (hopefully) overwhelm the network. In the rest of this discussion, we assume that the flow control window is larger than cwnd, or that the network rather than the receiver is the performance bottleneck. Because it takes roughly one round-trip time (rtt) to transmit a window s worth of packets, the equivalent transmission rate for such a window-based protocol is the ratio of cwnd/rtt packets per second.

A different approach from window-based congestion control is rate-based congestion control. Here, there s no window to modulate the transmission rate (although there usually is a flow control window to ensure that the receiver buffer does not overrun). Unlike in window-based protocols where incoming ACKs "clock" data packets (we discuss this idea in more detail when we talk about TCP), a rate-based protocol uses an explicit timer to decide when packets are sent on the network. In fact, it isn't enough to simply use a rate in such protocols what s usually needed is both a burst size (number of packets or bytes) and a burst interval (over which the rate will be measured).

As described above, both schemes react to individual indications of congestion. An alternate approach is to monitor loss rate p and set a rate r as a function of p. We will look at this idea in Section 5.

4.1 Decrease and increase algorithms

We are now in a position to discuss the details of the decrease and increase algorithms. It is clear that there are an infinite number of ways to decrease and increase windows, and so we restrict ourselves to a simple, practical class of control algorithms called linear algorithms. Here, if wi is the value of cwnd in the ith round-trip since the start of the connection,

w_{i+1} = aw_i + b

for real-numbered constants and . and are of course different for the decrease and increase cases, and ensure that wi+1 is smaller than or greater than wi respectively.

A good way to view such controls is using phase plots (see the Chiu and Jain paper for an example). If there are n sources sharing the bottleneck, this plot has n axes we consider n = 2 for simplicity and clarity in the rest of our discussion. Each axis plots the window size (or rate) of a connection (wi), normalized to the maximum window (rate) value. Thus, w1 + w2 = 1 is the "optimal utilization line" and w1 = w2 is the "equi-fairness line." Points below the w1 + w2 = 1 line are when the network is under-utilized. The goal is to get to the intersection of the two lines if this happens then the system will have converged to an efficient and fair allocation.

Chiu and Jain show a neat result in this formalism under a synchronized feedback assumption that the occurrence of congestion is signaled to all connections sharing the bottleneck, an additive-increase, multiplicative-decrease or AIMD strategy converges to efficiency and fairness. Furthermore, no other linear control does.

The intuition behind this follows from the phase-plot picture. Additive increase improves fairness and efficiency, while multiplicative decrease moves the system from overload to under-utilization without altering fairness. Thus, no matter where we start in the phase plot, AIMD pushes the system to w1 = w2 = 0.5. In contrast, additive-decrease reduces fairness in the decrease phase while multiplicative-increase, multiplicative-decrease (MIMD) does not ever improve fairness.

One can show all this algebraically too, but the pictorial arguments are a lot more intuitive and prettier!

Thus, with AIMD, upon congestion

w_{i+1} = aw_i, 0 < a < 1

and while probing for excess capacity,

w_{i+1} = w_i + b, 0 < b

where wmax is the largest possible window size (equal to the product of end-to-end delay and bottleneck bandwidth). The intuition behind this is that when the network is congested, the queue lengths start to increase exponentially with time, and the decrease must be multiplicative to ensure that this dissipates quickly. The increase algorithm must move gingerly and also try to move toward fair allocations of bandwidth, which is achieved by an additive algorithm.

For example, TCP sets a = 0.5 and b = 1. That is, when the sender detects congestion, it reduces its window to half the current value, and when an entire round-trip goes by and all the packets were successfully received (as indicated by all the acks arriving safely), it increases the window by 1 packet, carefully probing for a little more bandwidth to use.

In general, this technique for increasing and decreasing windows is also called additive-increase/multiplicative-decrease (AIMD) congestion control. Pseudo-code for this algorithm using TCP s constants is shown below.

// On detecting congestion via packet loss,
cwnd := cwnd / 2;
/// When the current window is fully acknowledged (roughly one round-trip),
cwnd := cwnd + 1;

4.2 Conservation of packets

Good window-based congestion control protocols follow the conservation of packets principle, which says that for a connection "in equilibrium," the packet flow must be conservative (this is a term from fluid mechanics). A flow is in equilibrium if it is running with a full window of packets in transit, equal to the product of available bandwidth and delay (in the absence of any persistent queueing). Of course, the end-to-end transport layer knows neither the available bandwidth nor the non-queueing delay with any great accuracy, which is why it needs to estimate them via its increase algorithm and round-trip calculations. Note that it need not explicitly calculate either parameter for its congestion control, and can rely instead on the observation that in the absence of congestion, an entire window is acknowledged by the receiver in one round-trip duration. Then, the end-to-end congestion control protocol can ensure that packet conservation isn t violated by ensuring that a lost packet is not prematurely retransmitted (this would violate conservation because a packet presumed to be lost would still be in transit at the time of retransmission). An excessive number of premature retransmissions does in fact lead to congestion collapse, and this was the cause of the mid-19.0s disaster.

TCP uses a widely applicable principle called self-clocking to send packets. Here, the sender uses acks as an implicit clock to regulate new packet transmissions. Clearly, the receiver can generate acks no faster than data packets can reach it through the network, which provides an elegant self-clocking mechanism by which the sender can strobe new packets into the network.

Three possible reasons for packet conservation to not happen in a transport protocol

  1. Connection doesn't reach equilibrium.
  2. Premature injection of new packets by sender.
  3. Equilibrium can t be reached because of resource limits along path (or wild oscillations occur).

4.3 How TCP works: slow start, congestion avoidance and control

We now have sufficient background to use the above principles and apply them to a real protocol, TCP.

We start by observing that packet conservation applies to a connection in equilibrium, one which has reached its sweet-spot window size and is probing for more around it (and reacting to congestion using multiplicative decrease). But when a new connection starts, how does it know what this sweet spot is, and how does it get to it to get its ack-triggered "clock" going? TCP achieves this using a technique called slow start. Here, TCP sets its cwnd to 1 and sends one packett . When an ack arrives for it, it increases its window size by 1 and now sends 2 packets. Now, when an ack arrives for each of these packets , cwnd increases by 1. That is, cwnd is now equal to 4. In the slow start phase, cwnd increases by 1 for each incoming ack. In this phase, TCP uses the ack clock to not only clock packets out, but also to open up its window.

When do we stop this process? We would like to stop it when the window size reaches the equilibrium value (roughly the bandwidth-delay product), but of course we don t know what that is. So, we stop it at an estimate called the slow start threshold (ssthresh) or on packet loss (because that s a sign of congestion), after which we move into the linear increase phase described above. Of course, if we stop it due to congestion, cwnd multiplicatively decreases.

How fast does the window grow toward equilibrium in slow start? If the current cwnd is wi, the sender transmits wi packets within one rtt. If they all reach, wi acks come back to the sender, each of which increases cwnd by 1. So, after all wi packets are acknowledged (one rtt later), wi + 1 = 2wi. Thus, the window increase in slow start is multiplicative, increasing exponentially with time. Thus, slow start isn't really that slow, and reaches an equilibrium window size estimate W in log2W round-trips.

Once slow start completes, TCP uses the AIMD principle to adjust its cwnd. The only tricky part is in the increase pseudo-code, shown below.

// When an ack arrives for a packet,
cwnd := cwnd + 1/cwnd;

The 1/cwnd increment follows from our wanting to increase the congestion window by 1 after one round-trip. Because TCP wants the increase to be smooth rather than abrupt, we approximate it by increasing by 1/cwnd when each packet is acknowledged. When cwnd acks arrive, the effective increase is approximately 1, after about one round-trip. And cwnd acks would indeed arrive in the absence of congestion. This phase of TCP is also called the linear phase or congestion avoidance phase.

This also serves to point out the dual purpose of a TCP ACK-on the one hand, it is used to detect losses and perform retransmissions, and on the other, it is used to clock packet transmissions and adjust cwnd. Thus, any variability or disruption in the ACK stream will often lead to degraded end-to-end performance.

The final twist to TCP s congestion control algorithm occurs when a large number of losses occur in any window. Taking this as a sign of persistent congestion, TCP takes a long timeout before trying to resume transmission, in the hope that this congestion would clear up by then. At this point, it has obviously lost its self-clock, and so goes back into slow start to resume it sets its slow start threshold, ssthresh, to one-half of cwnd at the time this persistent congestion occurred. Finally, for each failed retransmission, TCP performs exponential backoff for successive retransmissions by doubling the interval between attempts, similar to the random exponential backoff retry technique used in the Ethernet9. One can show that with a large user population, you need at least an exponential backoff on congestion (or in the Ethernet case, collision) to be safe from congestion collapse.

Pseudo-code for TCP achieving its self-clock via slow start is shown below.

if (timeout) // connection in progress and many packets lost
ssthresh := cwnd/2; // set ssthresh on timeout
else { // connection start-up
ssthresh := MAX_WIN; // max window size, 64 KBytes or more
cwnd := 1;

Figure 3 The time evolution of TCP s congestion window.

We are now in a position to sketch the evolution of the congestion window, cwnd, as a function of time for a TCP connection (Figure 3). Initially, it starts at 1 and in slow start doubles every rtt, (A) until congestion occurs (B). At this point, if the number of lost packets isn t large, the sender halves its transmission rate (C), and goes into the linear increase phase probing for more bandwidth (D). This sawtooth pattern continues, until several packets are lost in a window and TCP needs to restart its self-clocking machinery (E). The process now continues as if the connection just started, except now slow start terminates when cwnd reaches ssthresh.

We conclude this section by observing that the soundness of TCP s congestion management is to a large degree the main reason that the Internet hasn't crumbled in recent years despite the phenomenal growth in the number of users and traffic. Congestion management is still a hot research area, with additional challenges being posed by Web workloads (large numbers of short flows) and by real-time audio and video applications for which TCP is an inappropriate protocol.

5 Throughput vs. loss-rate

A key result of the TCP AIMD mechanism is that the long-term TCP throughput \lambda is related to the loss rate p it observes. To first order, \lambda \propto 1/\sqrt{p}. This is sometimes called the "TCP-friendly equation" since flows that have a similar reaction to a static packet loss probability may compete well with TCP over long time scales.

Figure 4 The steady-state behavior of the TCP AIMD congestion window.

This result can be used for an "equation-based" approach to congestion control, where the sender uses receiver feedback to monitor an average loss rate, and set the transmission rate according to some function of the loss rate. To compete well with TCP, it would use the TCP formula (more exact versions of which, incorporating timeouts and other TCP details, have been derived).

One way of showing that the TCP throughput in steady-state varies as the inverse-square-root of the loss rate uses the "steady-state model" of TCP. Figure 4 shows this. Here, we assume one TCP connection going through the AIMD phases, losing a packet when the window size reaches Wm. Upon this loss, it assumes congestion, and reduces its window to Wm/2. Then, in each RTT, it increases the window by 1 packet until the window reaches Wm, whereupon a window-halving occurs. Consider the time period between two packet losses. During this time, the sender sends a total of \frac{W_m}{2}+(\frac{W_m}{2}+1)+(\frac{W_m}{2}+2)+...+(\frac{W_m}{2}+\frac{W_m}{2}) = \frac{3}{8}W_m^2 + O(W_m) packets. Thus, p=\frac{8}{3W_m^2}

Now, what s the throughput, \lambda, of such a TCP connection? Its window size varies linearly between \frac{W_m}{2} and Wm, so its average value is \frac{3}{4}W_m. Thus \lambda \approx \frac{3}{4} \frac{W_m}{RTT} packets/s.

These two expressions for p and \lambda imply that \lambda \propto \frac{1}{\sqrt{p}}

This result can also be shown for a simplistic Bernoulli loss model. In this model, we assume that each packet can be lost independently with a probability p and that even if multiple packets are lost in the same RTT (window), only one window-halving occurs. (This assumption is correct for modern TCP s, which consider congestion "epochs" of 1 RTT, so multiple losses within an RTT are considered to be due to the same congestion event.

In this model, suppose W(t) is the current window size. A short time \delta t later, W(t) can be one of two values, depending on whether a loss occurs (with probability p) or not (with probability 1 - p).

\begin{align*} W(t+\delta t) &= W(t) + \frac{\delta t}{W_t \cdot RTT} \; \;\; \; \; \mathrm{w.p}. 1-p\\ &= \frac{W(t)}{2} \; \;\; \; \; \; \; \; \mathrm{w.p.} p \end{align*}

This assumes a "fluid" model of packet and ACK flow. The \frac{1}{W} comes in because in additive-increase the window increases by 1 every RTT, and there are W ACKs in each RTT (assuming no delayed ACKs). What we want is the average value of the window, which is evolving stochastically according to the above equation. If the average is \bar{w}, then the drift in \bar{w} with time should be 0. This drift is equal to \frac{1-p}{\bar{w} - p\frac{\bar{w}}{2}. Setting this to 0 gives \(\bar{w} \propto \frac{1}{\sqrt{p}}.

6 Buffer sizing

An important problem in network design is figuring out how much packet buffering to put in the queues at each router. This is an important issue. Too little, and you don t accommodate bursts of packets very well. These bursts are commonplace in Internet traffic, in part because of the nature of applications, and in part because of the way in which TCP probes for spare capacity using its increase algorithms (slow start and additive-increase). On the other hand, too much buffering is both useless and detrimental to performance the only thing you get is longer packet delays and larger delay variations. (This is akin to long lines at amusement parks-they don t improve the throughput of the system at all and only serve to frustrate people by causing really long waits.)

It turns out that for TCP AIMD sources, a buffer size equal to the product of the link bandwidth and round-trip delay achives nearly 100% link utilization. To see this, consider a simple model of one TCP connection. Suppose the window size is Wm when it incurs a loss. Clearly, Wm = P + Q where P is the "pipe size" or bandwidth-delay product and Q is the queue size. P is the amount of data that can be outstanding and "in the network pipe" that does not sit in any queue.

Now, when congestion is detected on packet loss and the window reduces to \frac{W_m}{2} this round-trip sees a throughput of at most \frac{W_m}{2 \times RTT}. For 100% utilization, this should equal \mu, the link bandwidth. The smallest value of \frac{W_m}{2} that achieves this is \mu \times RTT, which means means that Q should be set to \mu \times RTT (i.e., P).

7 Other approaches

We now outline two other broad approaches to congestion control. The next two lectures deal with the first of these, router-based congestion control.

7.1 Router-based congestion control

Router-based solutions are implemented in the switches and routers in the network. The simplest form of router support for congestion control is displayed in FIFO (first-in-first-out) queues at routers, which forward packets in the same order they were received, and drop packets on overload. In such a network, most of the burden for congestion control is pushed to the end-hosts, with the routers providing only the barest form of congestion feedback, via packet drops. The advantage of this minimalism is that the interior of the network (i.e., the routers) don t need to maintain any per-connection state and are extremely simple. The onus for dealing with congestion is pushed to the sender, which uses packet losses as a signal that congestion has occurred and takes appropriate action.

This is essentially the approach taken in most of the Internet today most IP routers implement FIFO scheduling and drop-tail queue space management, dropping all packets from the "tail" of a full queue. This approach, of keeping the interior of the network largely stateless and simple, is in

keeping with the fundamental design philosophy of the Internet, and is one of the reasons for the impressive scalability of the Internet with increasing numbers of users and flows.

In recent years, a number of queue management algorithms have been proposed for deploying in Internet routers, and more will be developed in the future. or example, routers needn't wait until their queues were full before dropping packets. In the next lecture (L4), we will study some simple but effective ways in which routers can support and enhance end-to-end congestion control via good queue management (i.e., deciding when and which packets to drop).

In addition, a number of sophisticated scheduling algorithms, which segregate flows into different categories and arbitrate on which category gets to send the next packet, have also been developed in recent years. or instance, routers could segregate the packets belonging to each flow into separate queues and process them in a "fair" way, using a technique similar to round-robin scheduling (a variant of this, developed some years ago, is called fair queueing). This approach is of course significantly more complex than simple FIFO queueing, because it requires per-flow state and processing to handle packets. We will study fair queueing and some variants in a couple of lectures from now (L5). Note, however, that even if these scheduling and queue management schemes were widely deployed in the network, the end-to-end transport layer would still have to eventually deal with congestion and decide how to modulate its transmissions to avoid congestion to achieve high throughput and network efficiency.

7.2 Pricing

A radically different approach to network resource management would be to rely on pricing mechanisms and the associated economic incentives and disincentives to effect effcient and fair network use. Such approaches are based on the observation that bandwidth and queue space are simply economic commodities and that the economic principles of supply and demand ought to apply to them. Indeed, to some extent, telephone companies do control overload by variable-rate pricing (e.g., prices are cheaper during slack hours and more expensive during peak hours). or asynchronously multiplexed data networks, no one has really figured out how to do this and if it will work. In particular, while pricing is a promising mechanism to provision resources over sufficiently long time scales (e.g., weeks or months), relying exclusively on network service providers to deploy flexible pricing structures to keep up with the wide variation in traffic load over small time scales could be risky.

8 Summary

This lecture looked at some key issues in end-to-end congestion control. Congestion management at all levels-end-to-end, router-assisted, and pricing-based-remains an area of much active research.

Source: Hari Balakrishnan and MIT OpenCourseWare, https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-829-computer-networks-fall-2002/lecture-notes/L8e2ecc.pdf
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Last modified: Sunday, August 30, 2020, 3:38 PM