Question

Consider a connection that uses TCP Reno. The connection has an initial congestion window size of \(1 \mathrm{~KB}\), and an initial threshold of 64 . Assume that additive increase uses a step-size of \(1 \mathrm{~KB}\). What is the size of the congestion window in transmission round 8, if the first transmission round is number 0 ?

Short answer

The congestion window size at transmission round 8 is 66 KB.

Step-by-step solution

Understand the Initial Conditions

The initial congestion window size is given as \(1 \text{ KB}\), and the initial threshold is 64. In TCP Reno, when the congestion window is below the threshold, it is in slow start phase.

Calculate the Behavior of TCP Reno

In the slow start phase, the congestion window size doubles each round. Starting from \(1 \text{ KB}\) in round 0, it will be 2 KB in round 1, 4 KB in round 2, 8 KB in round 3, and continues doubling.

Determine the Phase Transition Point

The slow start will double until the window size hits the threshold of 64 KB. At which point, TCP Reno will switch to the congestion avoidance phase. This occurs at round 6 when the congestion window becomes 64 KB.

Apply Additive Increase

In the congestion avoidance phase, the congestion window size grows additively by a step-size of 1 KB per round. Thus, at round 7, the window size will be 65 KB.

Congestion Window Size at Round 8

Continuing the additive increase, the congestion window size will be 66 KB at round 8.

Key concepts

Congestion Window

The congestion window is a critical component of TCP Reno, playing a major role in controlling the amount of data that can be sent over a network before requiring an acknowledgment. It helps in managing the flow of data to avoid congestion on the network. The size of the congestion window typically starts small and grows over time, allowing it to adapt to network conditions.
In the context of TCP Reno, the congestion window begins with a small size, usually just enough for a single packet, which in this case is 1 KB. As TCP Reno progresses, this window size increases, allowing more data to be sent without waiting for acknowledgments. This dynamic adjustment is crucial in balancing efficient data transmission and network stability.
The congestion window size changes based on the phase TCP Reno is in: slow start or congestion avoidance. Each phase has distinct rules for how the window size should adjust to ensure data is sent efficiently and safely.

Slow Start Phase

The slow start phase is the initial stage of the TCP Reno algorithm designed to probe the network conditions and rapidly ramp up the size of the congestion window. In this phase, the congestion window increases exponentially, doubling its size every transmission round.
For instance, in the given exercise, starting with 1 KB, the window doubles each round: 2 KB in round 1, 4 KB in round 2, and so forth. This exponential growth continues until the window size reaches a predefined threshold, helping the connection to quickly utilize available bandwidth.
The purpose of the slow start phase is to quickly find the equilibrium point where the connection can comfortably send data without overwhelming the network, which can lead to packet loss.

Congestion Avoidance Phase

Once the congestion window reaches or exceeds the threshold, TCP Reno transitions into the congestion avoidance phase. In this mode, the increase in the congestion window's size is more conservative. Instead of doubling, the window grows linearly.
In the example provided, once the window hits the threshold of 64 KB at round 6, the growth changes to an additive increase of 1 KB per round, reflecting a cautious approach to maintaining network stability. This results in a window size of 65 KB in round 7 and 66 KB in round 8.
The shift from exponential to linear growth helps prevent congestion by ensuring the network isn’t overwhelmed, thus reducing the likelihood of packet loss and maintaining efficient data flow.

Threshold

The threshold in TCP Reno serves as a marker point that determines the transition between the slow start and congestion avoidance phases. It represents the congestion window size at which the algorithm switches from rapid to gradual window size increases.
In the exercise example, the threshold is set at 64 KB. When the congestion window meets this value during slow start, it indicates the network is nearing capacity, prompting a shift to slower, linear increases. This threshold is crucial for avoiding network congestion by managing how aggressively TCP Reno expands the congestion window.
In scenarios where network conditions change and congestion occurs, TCP Reno will adjust the threshold down, setting up a new limit for future phases, which helps in better managing the network load and keeping data flowing smoothly without losses.