Question

Suppose host A reaches host B via routers R1 and R2: A-R1-R2-B. Fast retransmit is not used, and A calculates TimeOut as \(2 \times\) EstimatedRTT. Assume that the A-R1 and \(R 2-B\) links have infinite bandwidth; the \(R 1 \longrightarrow R 2\) link, however, introduces a 1 -second-per-packet bandwidth delay for data packets (though not ACKs). Describe a scenario in which the R1-R2 link is not \(100 \%\) utilized, even though A always has data ready to send. Hint: Suppose A's CongestionWindow increases from \(N\) to \(N+1\), where \(N\) is R1's queue size.

Short answer

The R1-R2 link is underutilized due to queuing delay causing idle periods despite constant data readiness from A.

Step-by-step solution

- Understand the Problem

Review the network configuration and parameters given: A sends data to B via routers R1 and R2 with a 1-second delay on the R1 to R2 link for data packets (not for ACKs). Fast retransmit is not used, and TimeOut is calculated as twice the EstimatedRTT.

- Identify Key Components

Focus on the A-R1-R2 link configuration. This link has a critical 1-second bandwidth delay for data packets, while ACKs are instantaneous. The congestion window's behavior and link utilization are important.

- Congestion Window Dynamics

Suppose A's CongestionWindow increases from size N to N+1. This congestion window controls the number of packets in transit.

- Analyze the Queue Size Impact

Given R1's queue size is N, once the window size reaches N, R1 starts queuing packets. Increasing the window to N+1 means the queue is now overfilled, causing a delay.

- Calculate Effective Utilization

While data packets wait due to the queue, the R1-R2 link can go underutilized. This happens because the sender waits for ACKs to clear the congestion window and retransmit new data, impacting throughput.

- Scenario Explanation

Under this scenario, even though A always has data ready, the 1-second bandwidth delay and queue size interaction cause periods where the R1-R2 link is idle.

Key concepts

CongestionWindow

In Transmission Control Protocol (TCP), the CongestionWindow is a fundamental concept. It determines the number of data packets that a sender can send without receiving an acknowledgment (ACK) from the receiver. This window aims to optimize the flow of data, preventing congestion and ensuring efficient use of network resources.

Initially, the CongestionWindow is small. As the sender receives ACK packets, it gradually increases, allowing more data packets to be sent. However, it grows cautiously to avoid overwhelming the network. This mechanism not only manages the flow of data but also reacts to signs of congestion by adjusting the window size.

In our example, imagine the CongestionWindow increasing from N to N+1. If N equals the capacity of a router's queue, then N packets can be sent before packets need to start queuing, potentially causing delays.

Bandwidth Delay

Bandwidth delay is the time taken for data to travel from one point to another in a network. It is a combination of the bandwidth of the connection and the time delay incurred while data traverses the network.

In our scenario, the R1-R2 link incurs a significant 1-second-per-packet delay for data packets, though not for ACK packets. This disparity means that while data packets take time to transfer, acknowledgments arrive instantaneously.

This delay contributes to the overall round-trip time (RTT), affecting how fast the sender can react to network conditions and adjust the CongestionWindow, ultimately impacting how efficiently the network is utilized.

Queue Size

Queue size in a network router is the number of data packets that can be held in the router's memory buffer before the packets must wait to be transmitted. When the queue is full, additional incoming data packets have to wait, causing delays.

For instance, if router R1 has a queue size of N, it can temporarily store N packets. Any additional packets received when the queue is full must wait until there's space available, creating a bottleneck.

In our example, when A's CongestionWindow goes from N to N+1, R1's queue starts to overfill. The R1-R2 link could then underutilize its capacity as the sender A waits for acknowledgments to clear the way for sending new data, despite always having data ready to send.

Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP) is a core protocol of the Internet Protocol Suite. TCP ensures reliable, ordered, and error-checked delivery of data between applications running on hosts.

TCP controls how data is transmitted and ensures that it reaches its destination in the same order in which it was sent. Among its many mechanisms, TCP uses the CongestionWindow to manage the flow of data packets and avoid congestion.

TCP's congestion control algorithms, like slow start and congestion avoidance, smoothly adjust the data flow to maximize network performance while preventing overload.

Data Packet

A data packet is a formatted unit of data carried by packet-switched networks like the internet. In TCP, data packets are the core components transmitted from sender to receiver.

Each data packet contains a portion of the overall message being sent, along with metadata such as source, destination, and sequencing information, ensuring that packets can be properly reassembled in the correct order.

In our scenario, data packets going from A to B operate under the restrictions of the 1-second bandwidth delay on the R1-R2 link, while the corresponding acknowledgment packets face no such delay.

ACK Packet

An ACK packet (Acknowledgment packet) is used in TCP to confirm receipt of data packets. When the sender A sends data packets to receiver B, B sends back ACK packets to inform A that the data was received successfully.

This feedback mechanism ensures reliability in TCP communication. If the sender does not receive an ACK within a certain time frame, it assumes the packet was lost and retransmits it.

In our case, the ACK packets travel without the 1-second delay on the R1-R2 link, enabling the sender to quickly update its CongestionWindow and manage the flow of data efficiently, as long as the link is not under a constraint imposed by an overfilled queue at R1.