CS6250 CS 6250 Exam 1 Latest

CS6250 / CS 6250 Exam 1 (Latest Update 2025 / 2026) Computer Networks | Questions & Answers | Grade A | 100% Correct - Georgia Tech

Question:

What is transmission control and why do we need to control it?

Answer:

is a mechanism in the transport layer to control the transmission rate. It is used to determine and adapt the transmission rate given the changing host and network conditions.UDP lets the application developers implement the mechanisms for transmission control, while TCP handles it for the user, and deals with issues like fairness in using the network.

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Question:

What is flow control and why do we need to control it?

Answer:

is a mechanism used to dynamically control the sender's transmission rate based on the receivers' buffer availability (called receive window) to protect the receiver's buffer. This avoids overflowing the receiver's buffer.

Question:

What is congestion control?

Answer:

is a mechanism used to dynamically control the sender's transmission rate to avoid congestion in the network (longer queues, packet drops, etc.).

Question:

What are the goals of the congestion control?

Answer:

• Efficiency. High throughput, or network utilization.

• Fairness. Each user should have its fair share (we will assume, equal

bandwidth) of the network bandwidth.

• Low delay. High throughput (with large buffers) would lead to long queues

in the network leading to delays. Applications that are sensitive to network 2 / 4

delays such as video conferencing will suffer. Thus, we want the network delays to be small.

• Fast convergence. A flow should be able to converge to its fair allocation

fast, so that even short flows will get their fair share of the network.

Question:

What is network-assisted congestion control?

Answer:

We rely on the network layer to provide explicit feedback to the sender about congestion in the network. For instance, routers could use ICMP source quench to notify the source that the network is congested. However, under severe congestion, even the ICMP packets could be lost, rendering the network feedback ineffective

Question:

What is end-to-end congestion control?

Answer:

As opposed to the previous approach, the network here does not provide any explicit feedback about congestion to the end hosts. Instead, the hosts infer congestion from the network behavior and adapt the transmission rate. This largely aligns with the end-to-end principle adopted in the design of the networks

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Question:

How does a host infer congestion?

Answer:

• Through packet delay.

As networks congest, queues in the router buffers build up. As packet round trip times increase (estimated based on ACKs) can be an indicator of congestion in the network, but packet delays tend to be variable, so it is not an straight forward indicator.

• Through packet loss.

As the network congests, routers start dropping packets. Packets can be lost due to other reasons such as routing errors, hardware failure, TTL expiry, error in the links, or flow control problems. Early implementations of TCP used packet loss as a signal for congestion.

Question:

How does a TCP sender limit the sending rate?

Answer:

A TCP sender cannot send faster than the slowest component, which is either the network or the receiving host. A sender uses ACKs as a pacing mechanism. TCP uses a congestion window which is similar to the receive window used for flow control. It represents the maximum number of unacknowledged data that a sending host can have in transit (sent but not yet

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