Question

Suppose that the TCP congestion window is set to \(18 \mathrm{~KB}\) and a timeout occurs. How big will the window be if the next four transmission bursts are all successful? Assume that the maximum segment size is \(1 \mathrm{~KB}\).

Short answer

The window size after four successful transmission bursts will be \(16 \mathrm{~KB}\).

Step-by-step solution

Understanding TCP Window Reduction on Timeout

When a TCP timeout occurs, the congestion window is reduced as part of the congestion control mechanism. Typically, this reduction will set the congestion window size to the maximum segment size (MSS), which is given as \(1 \mathrm{~KB}\). Therefore, after the timeout, the congestion window will be reset to \(1 \mathrm{~KB}\).

Describing Slow Start Phase

After the window size reduces, TCP enters the slow start phase. In this phase, the congestion window size doubles every Round Trip Time (RTT) until it reaches a threshold. This doubling is because upon each successful transmission burst, TCP increases the window size by one MSS for every acknowledged packet during slow start.

Transmission Burst 1

After the first successful transmission burst, the window size doubles from \(1 \mathrm{~KB}\) to \(2 \mathrm{~KB}\), as each successful RTT allows the window to grow from \(1 \) to \(1 + 1 = 2 \).

Transmission Burst 2

With the second successful transmission, the window doubles from \(2 \mathrm{~KB}\) to \(4 \mathrm{~KB}\). This growth is due to the accrued acknowledgments from the successful transmission.

Transmission Burst 3

The third successful transmission sees the window size increase from \(4 \mathrm{~KB}\) to \(8 \mathrm{~KB}\), effectively doubling again as part of TCP's slow start algorithm.

Transmission Burst 4

Finally, with the fourth successful transmission burst, the window doubles from \(8 \mathrm{~KB}\) to \(16 \mathrm{~KB}\). This is the result of the continued exponential growth during the slow start.

Key concepts

TCP Timeout

A TCP timeout is an important mechanism in TCP congestion control which kicks in when the acknowledgment for a sent segment is not received within a specific period. When a timeout happens, it indicates potential congestion in the network. As a response, TCP drastically reduces the congestion window, typically resetting it to the size of one Maximum Segment Size (MSS). This reduction helps to avoid overwhelming the network by sending too many packets at once. The intention is to ensure that the sender does not contribute further to the congestion, and allows the network to recover.

Slow Start Phase

Once the congestion window is reduced, TCP goes into the slow start phase. Slow start is a fundamental mechanism that controls the growth of the congestion window. During this phase, the congestion window size doubles every Round Trip Time (RTT). This means after every successful transmission burst, the size increases exponentially.

• The reason for this exponential increase is that for every packet acknowledged, the window grows by one MSS.
• Initially, the growth is slow, which is why it's referred to as 'slow start.'
• The aim is to quickly increase the transmission rate to find the network's current capacity.

Once a certain threshold is reached, this aggressive growth slows down to prevent congestion.

Congestion Window

The congestion window is a crucial part of TCP's congestion control. It determines the number of unacknowledged packets that can be sent over the network. The window size affects how much data TCP can send before needing an acknowledgment. During a timeout, this window reduces significantly to curb the risk of exacerbating congestion. As the network stabilizes and successful transmissions occur, the window size grows. Initially, it grows exponentially during the slow start phase, doubling with each successful RTT, but later the growth becomes more cautious to prevent network overload.

Round Trip Time (RTT)

Round Trip Time (RTT) is the time taken for a packet to travel from the sender to the receiver and back again. In TCP congestion control, RTT is vital in determining window growth.

• The slow start phase relies on RTT for the timing with which the congestion window size is adjusted. After a complete RTT, the window size may double during the slow start, given successful delivery.
• RTT estimates also influence the timeout settings, as they help TCP set realistic expectations for acknowledgments.
• Monitoring and accurately estimating RTT allows TCP to better manage congestion and optimize data transfer across the network.

Understanding RTT is essential for network efficiency and minimizing the impact of congestion.