Question

True or false? Consider congestion control in TCP. When the timer expires at the sender, the value of ssthresh is set to one half of its previous value.

Short answer

True, TCP sets ssthresh to half its previous value when the timer expires.

Step-by-step solution

Understanding TCP Congestion Control

TCP uses a congestion control algorithm to manage data transmission over the network. It employs several strategies, including Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery. Among these, the concept of 'ssthresh' (slow start threshold) is crucial as it determines when TCP exits the Slow Start phase and enters the Congestion Avoidance phase.

Analyzing Timer Expiration in TCP

When a timeout occurs in TCP, it indicates packet loss, which may be due to congestion. In response, the TCP congestion control algorithm must react to prevent further congestion.

TCP Action on Timeout

Upon timer expiration, TCP performs the following actions: - It sets the congestion window (\(cwnd\)) to 1 Maximum Segment Size (MSS) to restart the Slow Start phase.- It updates the slow start threshold (\(ssthresh\)).- The \(ssthresh\) is set to half of the current size of the congestion window before the timeout occurred.

Verifying the Statement

According to the step 3 description, when the timer expires, TCP does indeed set \(ssthresh\) to one half of its current value prior to the timeout, considering the protocol's standard behavior for handling congestion. Therefore, the statement is true.

Key concepts

Slow Start Threshold (ssthresh)

In TCP congestion control, the **slow start threshold**, or **ssthresh**, is a critical parameter that manages network traffic. It defines the boundary between the Slow Start phase and the Congestion Avoidance phase. Initially, when a connection is established, TCP begins the Slow Start phase. In this phase, the sending rate starts slow and exponentially increases until it hits the ssthresh value.

Once ssthresh is reached, TCP moves to the Congestion Avoidance phase, where the sending rate grows linearly. This gradual increase ensures the network avoids congestion by not overwhelming the network with a sudden burst of data.

In the event of a timeout, indicating potential congestion, TCP adjusts the ssthresh to prevent similar issues. Specifically, TCP sets ssthresh to half of the congestion window size before the timeout. This adjustment reflects a conservative approach to managing traffic flow in the network, enabling a smoother transition back to efficient data transmission. By using ssthresh effectively, TCP maintains a balance between congestion control and efficient data delivery.

TCP Timeout Handling

When a TCP timeout occurs, it suggests that packets have either been lost or delayed, often because of network congestion. This signals the network transmission to take preventive measures to avoid further congestion problems. A **timeout** serves as a mechanism for TCP to detect congestion and trigger adjustments in the data flow.

Upon detecting a timeout, TCP performs key actions to mitigate congestion issues. These include reducing the congestion window size to one Maximum Segment Size (MSS) and recalibrating the ssthresh to half the previous congestion window size before timeout. The drastic reduction in the congestion window size ensures that the network gradually returns to stability by entering the Slow Start phase again.

Overall, TCP timeout handling is a proactive approach that helps maintain an optimal balance between data transmission efficiency and network congestion prevention. This delicate balance is crucial to ensure reliable communication in dynamic network environments, preventing excessive packet loss and delays.

Congestion Window (cwnd) Management

The **congestion window**, or **cwnd**, is an essential component in TCP's congestion control algorithm, managing the amount of data that can be sent before receiving an acknowledgment. The size of the cwnd directly impacts the flow of data in the network, reflecting the current network conditions.

Initially, during the Slow Start phase, the cwnd begins at a low value and grows exponentially to rapidly probe the available bandwidth. Once it reaches the ssthresh, the rate of growth changes to linear, indicating the transition to Congestion Avoidance. This linear growth allows TCP to maximize data throughput while minimizing the risk of congestion.

In the case of a timeout or packet loss, TCP employs a conservative approach by reducing the cwnd size to one MSS, effectively resetting the Slow Start. This action helps TCP assess the network conditions once more and adjust the data flow rate accordingly. Proper management of the cwnd is crucial to avoiding excessive network congestion while ensuring efficient data transmission.