Computer Networks · free preview

Reliable Transmission

Frames are corrupted or lost, so reliable delivery is built from acknowledgments and timeouts in the ARQ family, stop-and-wait is the simplest member and wastes most of a long fat link, sliding window keeps the pipe full by bounding outstanding frames with a send window and a receive window governed by cumulative acknowledgments, finite sequence numbers force the window to stay below half the sequence space, and the ARPANET's concurrent logical channels show that even stop-and-wait can fill a link when multiplied.

1. Acknowledgments and timeouts

Acknowledgments and timeouts

2. Stop-and-wait and the price of waiting

Stop-and-wait and the price of waiting

3. Sliding window

Sliding window

4. Another way to fill the pipe

Another way to fill the pipe

5. The acknowledgment ambiguity

In stop-and-wait, a frame arrives intact but its acknowledgment is lost. What failure follows without sequence numbers?

6. Pricing stop-and-wait

On a 1.5 megabit per second link with a 45 millisecond RTT, stop-and-wait sends one 1 kilobyte frame per round trip. What sending rate results?

7. The sender's invariant

Which invariant must the sliding window sender always maintain?

8. Cumulative acknowledgments

A receiver with window 4 has last acknowledged frame 5. Frames 7 and 8 arrive, then frame 6. What happens?

9. The half-space rule

With RWS equal to SWS, why must the window be strictly less than half the sequence number space?

10. Concurrent logical channels

How did the ARPANET link protocol keep a link full while using only stop-and-wait?

11. The timeout's partner

ARQ combines timeouts with small control frames the receiver sends back to confirm arrival. What are these control frames called?

12. The one-bit fix

Stop-and-wait distinguishes a retransmitted frame from the next new frame using a sequence number field. How many bits wide is that field?

13. The cumulative rule

In sliding window, what kind of acknowledgment covers every frame up to the acknowledged number, even if some higher frames arrived earlier?

14. The window bound

With the receive window equal to the send window, the send window must stay strictly below half of what quantity, raised by one?

Practice this interactively

Free account · instant grading · spaced review that schedules itself.

Start this course for free