Reliable Delivery over an Unreliable Network · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Making packets behave

Computing · Networks & Security · ages 18-19
Name ______________________   Date ____________

  1. Which note tells the sender that bytes arrived safely?

    • A duplicate
    • An acknowledgement
    • A window

  2. Two packets of one message take different routes, and one route is faster. What happens?

    • They arrive out of order
    • They merge into one packet
    • The slower one is deleted

  3. In TCP, every byte carries a sequence number so the receiver can restore the order.

    Circle one:   True   False

  4. A sender ships segments 1 through 6. Acknowledgements confirm bytes through segment 3. How many segments still need confirming?

    Answer: ______________

  5. A segment goes missing mid-transfer. What triggers its resend?

    • The router rebuilds the missing bytes
    • The application asks the user to resend
    • The sender's timer runs out with no acknowledgement for it

  6. One packet of a live video call goes missing. Why does the app skip it instead of resending?

    • A resend would arrive too late to use
    • UDP stores every packet forever
    • Lost video packets rebuild themselves

  7. Two links carry the same data, but one has a much longer delay. Why does a bigger window help on the slow one?

    • It removes the delay
    • It makes each packet travel faster
    • It keeps a whole batch in flight instead of waiting per packet

  8. Segments arrive in the order 3, 1, 2. How does the receiver hand up the correct message?

    • It plays them in arrival order
    • It reads the sequence numbers and rebuilds 1, 2, 3
    • It drops all three and waits
LightMySky · lightmysky.comW1-mt_kCM411CSac-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Making packets behave W1-mt_kCM411CSac-s1

  1. An acknowledgement · Acknowledgements name the bytes now safely arrived.
  2. They arrive out of order · Different routes mean different travel times, so arrivals shuffle.
  3. True · Numbers mark each byte's place for reassembly.
  4. 3 · Segments 4, 5 and 6 are still unconfirmed.
  5. The sender's timer runs out with no acknowledgement for it · A missing note at timeout means the piece counts as lost.
  6. A resend would arrive too late to use · For live pictures a quick gap beats a slow catch-up.
  7. It keeps a whole batch in flight instead of waiting per packet · Batching keeps data flowing while replies travel back.
  8. It reads the sequence numbers and rebuilds 1, 2, 3 · Numbers beat arrival order every time.
Worksheet · LightMySky