Sliding Window Protocols

Keep several packets in flight, recover from the ones the network eats. Drop any packet or ACK and see what each protocol resends.

Window: 4

RTT: 6 ticks

Timeout: 8 ticks

Animation Speed: 700ms per tick

Tick 0 · step 1 of 32sent 0 · resent 0

acknowledged / deliveredsent, awaiting ACKqueued to resendin window, not sentbuffered (SR)

10 packets to send. The sender may have up to 4 unacknowledged packets in flight. Each one takes 3 ticks to cross the link, so a round trip is 6 ticks; the retransmission timer is 8.

Inject Losses

Presets

Drop packet

Drop its ACK

Each marked packet or ACK is lost on its first trip only; the retransmission gets through.

Same Losses, Three Protocols

protocoltickssentresent

Stop-and-Wait69111

Go-Back-N31144

Selective Repeat26111

Stop-and-Wait wastes the pipe; Go-Back-N fills it but pays for a lost packet with a whole window of resends; Selective Repeat resends only what was lost, at the cost of a receive buffer and a timer per packet. Try dropping only an ACK: a later cumulative ACK covers it for Go-Back-N, while Selective Repeat has to resend.

How They Differ

Window1NN

Receiver buffernonenoneN packets

ACKsper packetcumulativeper packet

Timers11 (oldest)1 per packet

On timeout resendthat packetall outstandingthat packet

Knowledge Check

Question 1 of 3

Go-Back-N with window 4 has packets 2, 3, 4 and 5 outstanding. Packet 2 is lost; 3, 4 and 5 arrive. What happens when packet 2's timer expires?

DSAverse

Sliding Window Protocols

Inject Losses

Same Losses, Three Protocols

How They Differ

Knowledge Check

See also

Sliding Window Protocols

Inject Losses

Same Losses, Three Protocols

How They Differ

Knowledge Check

See also