Question

In this problem, we consider the delay introduced by the TCP slow-start phase. Consider a client and a Web server directly connected by one link of rate \(R\). Suppose the client wants to retrieve an object whose size is exactly equal to \(15 S\), where \(S\) is the maximum segment size (MSS). Denote the round-trip time between client and server as RTT (assumed to be constant). Ignoring protocol headers, determine the time to retrieve the object (including TCP connection establishment) when a. \(4 S / R>S / R+R T T>2 S / R\) b. \(S / R+R T T>4 S / R\) c. \(S / R>R T T\).

Short answer

a) 5 RTTs; b) 16 RTTs; c) 5 RTTs.

Step-by-step solution

Understand the TCP Slow-Start Mechanism

TCP uses a congestion control algorithm known as slow start, meaning it initially sends data slowly to avoid network congestion. It starts with a congestion window (cwnd) of 1 MSS, then increases exponentially until a threshold or network capacity is met. Each RTT allows the window size (cwnd) to increase, sending 1 MSS, 2 MSS, 4 MSS, etc.

Establish Timing for Full Object Retrieval

To find the time to download the 15 MSS object, we need to consider both connection establishment and data transfer under TCP slow-start. Establishment requires 1 RTT (Client sends SYN, Server replies with SYN+ACK, Client sends ACK). Data transfer will be calculated differently based on the given conditions.

Construct Scenario for Condition a

Condition a: Start by establishing the connection in 1 RTT. Following the exponential growth, the TCP window opens exponentially after each RTT as follows: RTT 1 - sends 1 MSS, RTT 2 - sends 2 MSS, RTT 3 - sends 4 MSS, RTT 4 - sends 8 MSS (total 15 MSS sent in 4 RTTs). Thus, total time required is 5 RTTs since 1 RTT is for connection establishment.

Construct Scenario for Condition b

Condition b suggests a network delay where the size of one MSS is relatively small compared to outbound bandwidth delay. In the worst case depicted here, the server can only send 1 MSS per RTT due to higher latency compared to sending rate (bandwidth). The object requires 15 MSS, so we need 15 RTTs plus 1 RTT for setup, totaling 16 RTTs.

Construct Scenario for Condition c

For condition c, where the size of one MSS is larger than the RTT, the TCP connection setup takes an initial RTT. Once started, the full bandwidth is utilized efficiently after initiation. The TCP window behaves differently, sending more segments sooner due to small RTT. It can send the entire object quickly. Required RTT for 15 MSS is within 4 rounds given the initial exponential increase, assuming complete bandwidth utilization in fewer RTTs. Thus, requires 5 RTTs in total (4 for data, 1 for setup).

Key concepts

Congestion Control

TCP congestion control is an essential mechanism that helps prevent network congestion by regulating the amount of data the sender can send into the network before requiring an acknowledgment. This process ensures that networks remain stable and data is delivered efficiently.

The key feature of TCP's congestion control is the slow-start phase. Initially, when a connection begins, TCP starts by sending a small amount of data (1 MSS or Maximum Segment Size). Once the data is acknowledged, the congestion window (cwnd) doubles. This exponential growth continues until a threshold is reached or an acknowledgment is delayed, indicating potential congestion.

• In a stable network, this helps to achieve optimal data flow and prevent packet loss.
• During this phase, TCP adapts to network conditions smoothly, trying to stay just below the congestion threshold.
• Slow-start is crucial for maintaining equilibrium in bandwidth usage across networks.

Understanding this concept is key to optimizing network performance and ensuring that TCP remains responsive to varying network environments.

Round-Trip Time (RTT)

Round-trip time, often abbreviated as RTT, is a critical metric in networking that measures the time it takes for a data packet to travel from a source to a destination and back again. This includes the delay caused by both the transmission and processing in the network.

RTT is pivotal during the TCP slow-start phase as it influences how quickly the congestion window can expand. The faster the RTT, the quicker the sender can increase the data rate.

• A shorter RTT indicates quicker responsiveness and reduces waiting time for acknowledgments.
• Conversely, a longer RTT can result in slower data transmission rates due to increased waiting time for acknowledgments.
• Monitoring RTT helps in optimizing TCP flow control, as adjustments can be made to improve speed and reduce congestion.

By continuously measuring RTT, TCP can dynamically adjust its transmission strategy to match real-time network conditions, optimizing data flow effectively.

Maximum Segment Size (MSS)

The Maximum Segment Size or MSS is a concept representing the largest segment of data that a device can accommodate in a single transport layer operation. In TCP, MSS is a critical factor as it determines the maximum amount of data that can be sent in each packet.

Understanding MSS is essential for efficient network communication. When setting the MSS, there are a few considerations:

• The MSS is negotiated during the TCP connection establishment and influences the TCP slow-start process.
• Choosing an MSS that is too small can lead to inefficient use of bandwidth, as more packets will be required to send the same amount of data.
• If the MSS is too large, it might lead to fragmentation in the network, which can reduce performance and increase overhead.

Properly configuring the MSS helps to ensure that networks can transmit data efficiently, balancing between minimizing overhead and maximizing throughput. This balancing act is critical for maintaining optimal network performance and resource usage. Understanding the MSS and its role in TCP can lead to better network design and performance tuning.