Flow control (stop-and-wait, sliding window) in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of flow control in data communication?

Flow control ensures that the sender does not overwhelm the receiver by sending data faster than it can be processed, preventing data loss.

beginner

Explain the Stop-and-Wait flow control method.

In Stop-and-Wait, the sender sends one frame and waits for an acknowledgment before sending the next frame. This ensures reliable delivery but can be slow.

intermediate

What is the main disadvantage of Stop-and-Wait flow control?

It is inefficient for high-speed or long-distance networks because the sender must wait for an acknowledgment after each frame, causing delays.

intermediate

Describe the Sliding Window flow control technique.

Sliding Window allows the sender to send multiple frames before needing an acknowledgment, using a 'window' size to control how many frames can be sent at once, improving efficiency.

intermediate

How does the Sliding Window method improve over Stop-and-Wait?

Sliding Window increases data flow by allowing multiple frames to be sent without waiting for individual acknowledgments, reducing idle time and increasing throughput.

What does the sender do in Stop-and-Wait flow control after sending a frame?

A Sends all frames at once

B Waits for an acknowledgment before sending the next frame

C Sends the next frame immediately

D Sends a negative acknowledgment

In Stop-and-Wait, the sender waits for an acknowledgment after each frame before sending the next one.

Which flow control method allows sending multiple frames before receiving acknowledgments?

A Sliding Window

B Checksum

C Stop-and-Wait

D Parity Check

Sliding Window lets the sender send several frames within a window size before waiting for acknowledgments.

Why is Stop-and-Wait inefficient for long-distance communication?

A Because it sends too many frames at once

B Because it uses too large a window size

C Because it waits for acknowledgment after each frame, causing delays

D Because it does not check for errors

Waiting for acknowledgment after each frame causes delays especially when the round-trip time is long.

In Sliding Window, what controls how many frames can be sent before acknowledgment?

A Timeout value

B Frame size

C Packet priority

D Window size

The window size determines how many frames the sender can transmit before needing an acknowledgment.

Which of the following is a benefit of Sliding Window over Stop-and-Wait?

A Higher throughput

B Lower memory usage

C Simpler implementation

D No need for acknowledgments

Sliding Window improves throughput by allowing multiple frames to be sent without waiting for each acknowledgment.

Explain how Stop-and-Wait flow control works and mention one advantage and one disadvantage.

Think about sending one message at a time and waiting for a reply.

Describe the Sliding Window flow control method and how it improves data transmission efficiency.

Imagine sending several letters before waiting for a reply.

Practice

1. Which flow control method sends one packet and waits for an acknowledgment before sending the next?

easy

A. Stop-and-Wait

B. Sliding Window

C. Multiplexing

D. Routing

Solution

1. Step 1: Understand Stop-and-Wait flow control

   Stop-and-Wait sends one packet and waits for an acknowledgment before sending the next packet.

2. Step 2: Compare with other methods

   Sliding Window sends multiple packets before waiting, while Multiplexing and Routing are unrelated to flow control.

3. Final Answer:

   Stop-and-Wait -> Option A

4. Quick Check:

   One packet at a time = Stop-and-Wait [OK]

Hint: One packet then wait means Stop-and-Wait [OK]

Common Mistakes:

• Confusing Sliding Window with Stop-and-Wait
• Thinking Multiplexing is a flow control method
• Mixing routing with flow control

2. Which of the following correctly describes the sliding window flow control method?

easy

A. Sends one packet and waits for acknowledgment

B. Sends multiple packets before waiting for acknowledgment

C. Sends packets without any acknowledgment

D. Sends packets only after a timeout

Solution

1. Step 1: Recall sliding window behavior

   Sliding Window sends multiple packets before waiting for acknowledgment to improve speed.

2. Step 2: Eliminate incorrect options

   Sends one packet and waits for acknowledgment describes Stop-and-Wait. The other options do not describe standard flow control methods.

3. Final Answer:

   Sends multiple packets before waiting for acknowledgment -> Option B

4. Quick Check:

   Multiple packets sent = Sliding Window [OK]

Hint: Multiple packets sent before wait means Sliding Window [OK]

Common Mistakes:

• Choosing Stop-and-Wait for sliding window question
• Thinking no acknowledgment is used
• Confusing timeout with acknowledgment wait

3. Consider a sliding window protocol with window size 4. If the sender has sent packets 1, 2, 3, and 4, but only packets 1 and 2 are acknowledged, which packets can the sender send next?

medium

A. Packets 5 and 6

B. Packets 3, 4, and 5

C. Packets 5, 6, 7, and 8

D. Packets 3, 4, 5, and 6

Solution

1. Step 1: Understand sliding window acknowledgments

   Packets 1 and 2 are acknowledged, so the window slides forward by 2 packets.

2. Step 2: Calculate new window range

   Window size is 4, so after acknowledging 1 and 2, sender can send packets 5 and 6 (next two packets).

3. Final Answer:

   Packets 5 and 6 -> Option A

4. Quick Check:

   Window slides by acknowledged packets = Packets 5 and 6 [OK]

Hint: Window slides by acknowledged packets count [OK]

Common Mistakes:

• Trying to resend unacknowledged packets
• Sending too many packets beyond window size
• Not sliding window after acknowledgments

4. A stop-and-wait protocol implementation is not receiving acknowledgments and keeps resending the same packet. What is the most likely cause?

medium

A. The sender is sending multiple packets at once

B. The sender window size is too large

C. The receiver is not sending acknowledgments

D. The network is using sliding window instead

Solution

1. Step 1: Analyze stop-and-wait behavior

   Stop-and-wait waits for acknowledgment before sending next packet; no acknowledgment causes resending.

2. Step 2: Identify cause of no acknowledgment

   If acknowledgments are missing, likely the receiver is not sending them or they are lost.

3. Final Answer:

   The receiver is not sending acknowledgments -> Option C

4. Quick Check:

   No acknowledgment = Resend same packet [OK]

Hint: No ack means receiver not responding [OK]

Common Mistakes:

• Blaming sender window size in stop-and-wait
• Confusing sliding window with stop-and-wait
• Assuming multiple packets sent in stop-and-wait

5. In a sliding window protocol with window size 5, the sender transmits packets 1 to 5. Packets 2 and 3 are lost, but packets 1, 4, and 5 are acknowledged. Which packets should the sender retransmit next to maintain correct flow control?

hard

A. Packets 6 and 7

B. Packets 2, 3, and 6

C. Packets 2, 3, 4, and 5

D. Packets 2 and 3 only

Solution

1. Step 1: Identify lost packets

   Packets 2 and 3 are lost, so they need retransmission.

2. Step 2: Understand acknowledgments impact

   Packets 1, 4, and 5 are acknowledged, so only lost packets 2 and 3 must be resent before sending new packets.

3. Final Answer:

   Packets 2 and 3 only -> Option D

4. Quick Check:

   Retransmit lost packets only = Packets 2 and 3 [OK]

Hint: Retransmit only lost packets, not acknowledged ones [OK]

Common Mistakes:

• Retransmitting already acknowledged packets
• Sending new packets before lost ones
• Confusing lost packets with acknowledged packets