CONGESTION CONTROL
Congestion control refers to techniques and mechanisms that can either prevent congestion, before it happens, or remove congestion, after it has happened. In general, we can divide congestion control mechanisms into two broad categories: open-loop congestion control (prevention) and closed-loop congestion control (removal) as shown in Fig. below:
Open-Loop Congestion Control
In open-loop congestion control, policies are applied to prevent congestion before it happens. In these mechanisms, congestion control is handled by either the source or the destination.
Closed-Loop Congestion Control
Closed-loop congestion control mechanisms try to alleviate congestion after it happens.
Normally the sender window size is determined by the available buffer space in the receiver window (rwnd). In other words, we assumed that it is only the receiver that can dictate to the sender the size of the sender's window. We totally ignored another entity here-the network. If the network cannot deliver the data as fast as they are created by the sender, it must tell the sender to slow down. In other words, in addition to the receiver, the network is a second entity that determines the size of the sender's window.
Today, the sender's window size is determined not only by the receiver but also by congestion in the network.
The sender has two pieces of information: the receiver-advertised window size and the congestion window size. The actual size of the window is the minimum of these two.
$$\text{Actual window size=minimum (rwnd, cwnd)}$$
Congestion Policy
- TCP's general policy for handling congestion is based on three phases: slow start, congestion avoidance, and congestion detection. In the slow-start phase, the sender starts with a very slow rate of transmission, but increases the rate rapidly to reach a threshold.
- When the threshold is reached, the data rate is reduced to avoid congestion. Finally if congestion is detected, the sender goes back to the slow-start or congestion avoidance phase based on how the congestion is detected.
Slow Start, Exponential Increase: One of the algorithms used in TCP congestion control is called slow start. This algorithm is based on the idea that the size of the congestion window (cwnd) starts with one maximum segment size (MSS). The MSS is determined during connection establishment by using an option of the same name. The size of the window increases one MSS each time an acknowledgment is received. As the name implies, the window starts slowly, but grows exponentially.
As shown in the Figure below. Note that we have used three simplifications to make the discussion more understandable. We have used segment numbers instead of byte numbers (as though each segment contains only 1 byte). We have assumed that rwnd is much higher than cwnd, so that the sender window size always equals cwnd. We have assumed that each segment is acknowledged individually.
The sender starts with cwnd =1 MSS. This means that the sender can send only one segment. After receipt of the acknowledgment for segment 1, the size of the congestion window is increased by 1, which means that cwnd is now 2. Now two more segments can be sent. When each acknowledgment is received, the size of the window is increased by 1 MSS. When all seven segments are acknowledged, cwnd = 8.
Slow start cannot continue indefinitely. There must be a threshold to stop this phase. The sender keeps track of a variable named ssthresh (slow-start threshold). When the size of window in bytes reaches this threshold, slow start stops and the next phase starts. In most implementations the value of ssthresh is 65,535 bytes.
Congestion Avoidance, Additive Increase: If we start with the slow-start algorithm, the size of the congestion window increases exponentially. To avoid congestion before it happens, one must slow down this exponential growth. TCP defines another algorithm called congestion avoidance, which undergoes an additive increase instead of an exponential one.
When the size of the congestion window reaches the slow-start threshold, the slow-start phase stops and the additive phase begins. In this algorithm, each time the whole window of segments is acknowledged (one round), the size of the congestion window is increased by 1.
To show the idea, we apply this algorithm to the same scenario as slow start, although we will see that the congestion avoidance algorithm usually starts when the size of the window is much greater than 1. Figure below shows the idea.
Congestion Detection: Multiplicative Decrease If congestion occurs, the congestion window size must be decreased. The only way the sender can guess that congestion has occurred is by the need to retransmit a segment. However, retransmission can occur in one of two cases: when a timer times out or when three ACKs are received. In both cases, the size of the threshold is dropped to one-half, a multiplicative decrease. Most TCP implementations have two reactions:
If a time-out occurs, there is a stronger possibility of congestion; a segment has probably been dropped in the network, and there is no news about the sent segments.
In this case TCP reacts strongly:
1.1 It sets the value of the threshold to one-half of the current window size.
1.2 It sets cwnd to the size of one segment.
1.3 It starts the slow-start phase again.
If three ACKs are received, there is a weaker possibility of congestion; a segment may have been dropped, but some segments after that may have arrived safely since three ACKs are received. This is called fast transmission and fast recovery. In this case, TCP has a weaker reaction:
2.1 It sets the value of the threshold to one-half of the current window size.
2.2 It sets cwnd to the value of the threshold (some implementations add three segment sizes to the threshold).
2.3 It starts the congestion avoidance phase.
An implementations reacts to congestion detection in one of the following ways:
- If detection is by time-out, a new slow-start phase starts.
- If detection is by three ACKs, a new congestion avoidance phase starts.
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