Rdt2.0: channel with packet errors (no loss!) What mechanisms do we need to deal with error? Error detection Add checksum bits Feedback Acknowledgements (ACKs): receiver explicitly tells sender that packet received correctly Negative acknowledgements (NAKs): receiver explicitly tells sender that packet had errors Retransmission sender retransmits packet on receipt of NAK So, we need the following mechanisms: Error detection, Feedback (ACK/NACK), Retransmission 23
rdt2.0: FSM specification rdt_send(data) snkpkt = make_pkt(data, checksum) call from above sender ACK or NAK rdt_rcv(rcvpkt) && isnak(rcvpkt) receiver rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(nak) call from below rdt_rcv(rcvpkt) && isack(rcvpkt) Λ rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ack) 24
rdt2.0: Can this completely solve errors? call from above sender rdt_send(data) snkpkt = make_pkt(data, checksum) rdt_rcv(rcvpkt) && isack(rcvpkt) Λ ACK or NAK rdt_rcv(rcvpkt) && isnak(rcvpkt) What happens when ACK or NAK has errors? Approach: resend the current data packet? ACK error Duplicate packets The received packet is new or duplicate? 25
Handling Duplicate Packets Sender adds sequence number to each packet Sender retransmits current packet if ACK/ NAK garbled Receiver discards duplicate packet 26
rtd2.1: examples PKT(0) ACK error x PKT(0) ACK Receiver expects a pkt with seq. # 1 Duplicate pkt; discard it PKT(1) sender receiver 27
rdt2.1: sender, handles garbled ACK/NAKs rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isack(rcvpkt) Λ rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isnak(rcvpkt) ) rdt_send(data) sndpkt = make_pkt(0, data, checksum) call 0 from above ACK or NAK 1 rdt_send(data) ACK or NAK 0 call 1 from above rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isnak(rcvpkt) ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isack(rcvpkt) sndpkt = make_pkt(1, data, checksum) Λ Transport Layer 3-28
rdt2.1: receiver, handles garbled ACK/NAKs rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) sndpkt = make_pkt(ack, chksum) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ack, chksum) 0 from below 1 from below rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ack, chksum) Why? extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ack, chksum) Transport Layer 3-29
rdt2.1: summary for packet error Mechanisms for channel with packet errors Error detection, Feedback, Retransmission, Sequence# Sender: seq # added to pkt must check if received ACK/NAK corrupted Retransmit on NAK or corrupted feedback Receiver: must check if received packet is duplicate send NAK if received packet is corrupted Send ACK otherwise 30
rdt2.2: a NAK-free protocol Same functionality as rdt2.1, using ACKs only Instead of NAK, receiver sends ACK for last correctly received packet Receiver must explicitly include seq # of pkt being ACKed Duplicate ACK at sender results in same action as NAK: retransmit current pkt 31
rdt2.2: sender, receiver fragments rdt_rcv(rcvpkt) && (corrupt(rcvpkt) has_seq1(rcvpkt)) rdt_send(data) sndpkt = make_pkt(0, data, checksum) call 0 from above 0 from below ACK 0 sender FSM fragment receiver FSM fragment rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isack(rcvpkt,1) ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isack(rcvpkt,0) Λ rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ack1, chksum) Transport Layer 3-32
rdt3.0: channel with loss & packet errors What mechanisms do we need for packet loss? Timer! Sender waits reasonable amount of time for ACK (a Time-Out) If packet (or ACK) is just delayed (not lost): Retransmission will be duplicate, but use of seq. # s already handles this 33
rdt3.0 in action 34
rdt3.0 in action 35
Recap: Principles of Reliable Data Transfer What can happen over unreliable channel? What mechanisms for packet loss? Packet error, packet loss What mechanisms for packet error? Error detection, feedback, retransmission, sequence# Timeout! We built simple reliable data transfer protocol Real-world protocol (e.g., TCP) is more complex, but with same principles! 36
rdt3.0 sender rdt_rcv(rcvpkt) Λ call 0from above rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isack(rcvpkt,1) stop_timer timeout start_timer rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isack(rcvpkt,0) ) Λ Wait for ACK1 rdt_send(data) sndpkt = make_pkt(0, data, checksum) start_timer rdt_send(data) Wait for ACK0 call 1 from above sndpkt = make_pkt(1, data, checksum) start_timer rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isack(rcvpkt,1) ) Λ timeout start_timer rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isack(rcvpkt,0) stop_timer rdt_rcv(rcvpkt) Λ Transport Layer 3-37
Performance of rdt3.0 rdt3.0 works, but performance stinks example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet: T transmit = L (packet length in bits) R (transmission rate, bps) = 8kb/pkt 10**9 b/sec = 8 microsec U sender : utilization fraction of time sender busy sending U sender = L / R RTT + L / R =.008 30.008 = 0.00027 microsec 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link network protocol limits use of physical resources! Transport Layer 3-38
rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R sender receiver RTT first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R U sender = L / R RTT + L / R =.008 30.008 = 0.00027 microsec Transport Layer 3-39
Pipelined protocols Pipelining: sender allows multiple, in-flight, yet-to-beacknowledged pkts range of sequence numbers must be increased buffering at sender and/or receiver Two generic forms of pipelined protocols: go-back-n, selective repeat Transport Layer 3-40
Pipelining: increased utilization first packet bit transmitted, t = 0 last bit transmitted, t = L / R sender receiver RTT ACK arrives, send next packet, t = RTT + L / R first packet bit arrives last packet bit arrives, send ACK last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK Increase utilization by a factor of 3! U sender = 3 * L / R RTT + L / R =.024 30.008 = 0.0008 microsecon Transport Layer 3-41
Go-Back-N Sender: k-bit seq # in pkt header window of up to N, consecutive unack ed pkts allowed ACK(n): ACKs all pkts up to, including seq # n - cumulative ACK may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window Transport Layer 3-42
GBN: sender extended FSM rdt_send(data) Λ base=1 nextseqnum=1 rdt_rcv(rcvpkt) && corrupt(rcvpkt) if (nextseqnum < base+n) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ } else refuse_data(data) timeout start_timer Wait udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) udt_send(sndpkt[nextseqnum-1] ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) base = getacknum(rcvpkt)+1 If (base == nextseqnum) stop_timer else start_timer Transport Layer 3-43
GBN: receiver extended FSM default Λ expectedseqnum=1 Wait sndpkt = make_pkt(expectedseqnum,ack,chksum) rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(expectedseqnum,ack,chksum) expectedseqnum++ ACK-only: always send ACK for correctly-received pkt with highest in-order seq # may generate duplicate ACKs need only remember expectedseqnum out-of-order pkt: discard (don t buffer) -> no receiver buffering! Re-ACK pkt with highest in-order seq # Transport Layer 3-44
GBN in action Transport Layer 3-45
Selective Repeat receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received sender timer for each unacked pkt sender window N consecutive seq # s again limits seq #s of sent, unacked pkts Transport Layer 3-46
Selective repeat: sender, receiver windows Transport Layer 3-47
Selective repeat data from above : if next available seq # in window, send pkt timeout(n): resend pkt n, restart timer ACK(n) in [sendbase,sendbase+n]: sender mark pkt n as received if n smallest unacked pkt, advance window base to next unacked seq # receiver pkt n in [rcvbase, rcvbase+n-1] send ACK(n) out-of-order: buffer in-order: deliver (also deliver buffered, in-order pkts), advance window to next notyet-received pkt pkt n in [rcvbase-n,rcvbase-1] ACK(n) otherwise: ignore Transport Layer 3-48
Selective repeat in action Transport Layer 3-49
Selective repeat: dilemma Example: seq # s: 0, 1, 2, 3 window size=3 receiver sees no difference in two scenarios! incorrectly passes duplicate data as new in (a) Q: what relationship between seq # size and window size is safe? Transport Layer 3-50