Question

Consider the following two causes of a 1 -second network delay (assume ACKs return instantaneously): One intermediate router with a 1 -second outbound per-packet bandwidth delay and no competing traffic One intermediate router with a 100-ms outbound per-packet bandwidth delay and with a steadily replenished (from another source) 10 packets in the queue (a) How might a transport protocol in general distinguish between these two cases? (b) Suppose TCP Vegas sends over the above connections, with an initial CongestionWindow of 3 packets. What will happen to CongestionWindow in each case? Assume BaseRTT \(=1\) second and \(\beta\) is 1 packet per second.

Short answer

TCP Vegas will keep CongestionWindow around 3 packets for both routers but adjusting actively for the second one due to queue delays.

Step-by-step solution

- Understand the two scenarios for 1-second delay

First, identify the scenarios: 1. A router with a 1-second delay and no competing traffic. 2. A router with a 100-ms delay and 10 packets always in the queue.

- Analyze how a transport protocol distinguishes between the two cases

A transport protocol might distinguish the two cases by measuring Round Trip Time (RTT) or packet delays. In the first case, the delay will be consistently 1 second. In the second case, the delay will fluctuate around 1 second due to queue length fluctuations and the additional 100-ms delay.

- Assess the initial RTT for both cases

For the first router, the RTT would be consistently 1 second (since there's no competing traffic, RTT equals outbound delay). For the second router, initial RTT could vary but will be approximately 1 second due to the 100-ms delay plus the variable waiting time in the queue.

- Calculate CongestionWindow for TCP Vegas for the first router scenario

TCP Vegas measures RTT and adjusts CongestionWindow based on the difference between actual RTT and BaseRTT. Initial RTT = 1 second, BaseRTT = 1 second, so there’s no significant difference initially. CongestionWindow remains around 3 packets.

- Calculate CongestionWindow for TCP Vegas for the second router scenario

Initially, RTT might be slightly greater than BaseRTT due to queue delays. TCP Vegas observes this difference and would initially keep CongestionWindow around 3 packets but can adjust it based on subsequent RTT measurements since the queue delay might vary.

Key concepts

Round Trip Time (RTT)

Round Trip Time, or RTT, is the time it takes for a packet to travel from the sender to the receiver and back. It includes all types of delays:

• Propagation delay
• Transmission delay
• Queuing delay
• Processing delay

In network communications, RTT is a key metric for measuring the efficiency and performance. Simply put, a lower RTT indicates a faster and more responsive network. By constantly measuring RTT, transport protocols such as TCP adapt to changing network conditions. For example, if RTT increases, it might imply congestion or extended queuing delays. Conversely, a decrease could indicate improved network conditions. Understanding RTT helps protocols adjust their congestion control mechanisms to optimize data flow.

TCP Vegas

TCP Vegas is an enhancement over traditional TCP that places a greater emphasis on congestion avoidance rather than reacting to congestion. Unlike traditional protocols that rely on packet loss to signal congestion, TCP Vegas uses changes in RTT to detect congestion early. Here’s how it works:

• **BaseRTT:** The minimum RTT ever observed on the connection. Ideally, this is measured when the network is least congested.
• **Expected Throughput:** This is calculated based on the BaseRTT and current CongestionWindow size.
• **Actual Throughput:** This is measured in real-time based on the observed RTT and current CongestionWindow size.

TCP Vegas compares Expected Throughput with Actual Throughput. If the Actual Throughput is significantly lower than Expected Throughput, it means there might be congestion in the network. TCP Vegas adjusts the CongestionWindow size gradually, increasing it when throughput is good and decreasing it when congestion is detected. This proactive approach helps maintain a smoother and more stable network performance.

CongestionWindow

The CongestionWindow (CWND) is an essential part of TCP's congestion control algorithm. It dictates the number of packets a sender can send into the network before needing an acknowledgment from the receiver. Here’s how it impacts data flow:

• **Initial CWND:** TCP starts with a small CWND to probe the network's capacity.
• **Slow Start Phase:** Initially, CWND grows exponentially to quickly discover the available bandwidth.
• **Congestion Avoidance Phase:** Once a threshold is reached, CWND grows linearly to avoid congestion.
• **Congestion Detection:** If packet loss is detected, CWND is reduced to ease congestion.

TCP Vegas refines this mechanism by adjusting CWND based on RTT variations rather than solely on packet loss. For example, in our scenarios:
In the **first case**, with a consistent 1-second RTT, TCP Vegas maintains the CWND around 3 packets, as there’s no significant sign of congestion.
In the **second case**, where RTT might fluctuate slightly around 1 second due to queuing delays, TCP Vegas might make small adjustments to CWND to account for varying network conditions.