Question

In TCP Vegas, the calculation of ActualRate is done by dividing the amount of data transmitted in one RTT interval by the length of the RTT. (a) Show that for any TCP, if the window size remains constant, then the amount of data transmitted in one RTT interval is constant once a full window is sent. Assume that the sender transmits each segment instantly upon receiving an ACK, packets are not lost and are delivered in order, segments are all the same size, and the first link along the path is not the slowest. (b) Give a timeline sketch showing that the amount of data per RTT above can be less than CongestionWindow.

Short answer

If the window size remains constant, the amount of data transmitted in one RTT interval is constant. Practical conditions can result in data per RTT being less than the CongestionWindow.

Step-by-step solution

Title - Define Key Terms

Understand the key terms in the problem: RTT (Round-Trip Time), window size, ActualRate, ACK (Acknowledgment), and CongestionWindow. RTT is the time it takes for a data packet to travel from the sender to the receiver and back.

Title - Understand the Concept of Window Size

TCP transmission uses a window size (CongestionWindow) to control the amount of data sent before waiting for an ACK. If the window size remains fixed, the amount of data sent depends on this size.

Title - Amount of Data Transmitted in One RTT

For any TCP, if the window size remains constant, the sender sends out a window's worth of data, receives ACKs, and then sends out another window's worth of data. This means the total data sent per RTT interval is constant once the full window is sent. Let's call this amount WindowSize.

Title - Calculation of ActualRate

ActualRate is calculated as the amount of data sent in one RTT interval divided by the RTT. Since the amount of data sent in one RTT is constant at WindowSize, ActualRate = WindowSize / RTT.

Title - Sketching the Timeline

Draw a timeline of packet transmission and acknowledgments. Note that each window's worth of data sent out gets acknowledged within one RTT. If packets are not lost and are delivered in order, the sender sends out the next window worth of data as soon as ACKs are received.

Title - Data Per RTT Less Than CongestionWindow

Although the data sent per RTT is ideally equal to CongestionWindow, practical network conditions may result in the actual data sent being less than CongestionWindow. Factors like limited buffer space, network congestion, and temporary reductions in window size can cause this reduction.

Key concepts

TCP Congestion Control

TCP Congestion Control is a vital mechanism to manage traffic flow and prevent network congestion. It ensures efficient data transmission by adapting the rate of data flow based on network conditions. TCP Vegas leverages proactive congestion avoidance rather than merely reacting to packet losses. Instead of waiting for lost packets as a signal for congestion, TCP Vegas monitors Round-Trip Time (RTT) to detect signs of congestion early and adjust the sending rate accordingly. This proactive adjustment helps avoid packet loss and maintain smoother network performance.

Round-Trip Time (RTT)

Round-Trip Time (RTT) measures the time it takes for a data packet to go from the sender to the receiver and then back to the sender. It's a crucial metric in network performance, used extensively in TCP Vegas to gauge network conditions. TCP Vegas continuously monitors RTTs to predict potential congestion. If RTT increases, it may signify that the network is getting congested. In such cases, TCP Vegas will reduce the sending rate to prevent packet loss. Conversely, if RTT remains low, the network is likely clear, and the sending rate can be increased for better throughput.

Congestion Window

The Congestion Window is a control parameter within TCP that dictates the amount of data the sender can transmit before needing an acknowledgment (ACK) from the receiver. In TCP Vegas, the Congestion Window dynamically adjusts to network conditions. Initially, it starts small and grows as successful transmissions occur, following an additive increase approach. When potential congestion is detected (e.g., RTT increases), the Congestion Window shrinks to avoid overloading the network. This balancing act ensures that data transmission remains efficient without overwhelming network resources.

ActualRate

ActualRate in TCP Vegas is calculated as the amount of data sent in one RTT interval divided by the duration of RTT. Mathematically, it is expressed as: \[ ActualRate = \frac{WindowSize}{RTT} \]Where ‘WindowSize’ is the amount of data transmitted during one RTT interval. This value helps TCP Vegas determine whether to increase or decrease the sending rate. A smaller ActualRate compared to the expected rate indicates that the network is hauling more traffic, prompting a reduction in send rate to avoid potential congestion.

Acknowledgment (ACK)

The Acknowledgment (ACK) is a signal sent by the receiving device back to the sender to confirm receipt of data packets. In TCP, each successfully received packet triggers an ACK sent to the sender, facilitating continuous data flow. In TCP Vegas, the timely arrival of ACKs is critical. If ACKs are delayed, it suggests increasing RTT, signaling the network might be congested. The sender uses this feedback to adjust the send rate appropriately. Thus, ACKs not only confirm receipt but also provide real-time feedback on network conditions.