TCP over Wireless. Protocols and Networks Hadassah College Spring 2018 Wireless Dr. Martin Land 1

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1 TCP over Wireless Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 1

2 Classic TCP-Reno Ideal operation in-flight segments = cwnd (send cwnd without stopping) Cumulative ACK for each 2 or 3 segments cwnd > for most time near-continuous transmission Router Buffer TCP Buffer Finite buffers in routers Collision avoidance cwnd cwnd + 1 until buffers fill cwnd cwnd / 2 cwnd time Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 2

3 RTT and Buffer Errors Round Trip Time RTT = data transmit time + send buffer times + ACK transmit time + ACK buffer times Buffer time ~ typical service time buffer level RTT = random variable (rise / fall sharply) RTT Sender SEQ ACK Receiver Buffer time in TCP Timeout Buffer level RTT > RTO (retransmit timeout) Packet considered lost Out-of-order packet Buffer level RTT(packet k) > RTT(packet k+1) Receiver will send cumulative ACK if OOO packet not lost Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 3

4 RTT and Congestion Buffer error condition Sender Receiver Buffer level RTT > RTO for time T error Isolated error T error < time between packets Buffer empties before next packet No need to lower transmission rate RTT SEQ ACK Congestion T error > time between packets Multiple buffer errors Lower transmission rate to prevent buffer errors Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 4

5 Congestion Control Average transmission rate packets sent cwnd R = = time to ACK RTT R if cwnd or RTT Isolated buffer error T error < time between packets RTT R for T error Buffer corrects itself No need to change cwnd Congestion T error > time between packets RTT R Buffer still too full Senders: cwnd R Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 5

6 Classic TCP-Reno No Congestion in Collision Avoidance Sender Receiver ACK = 1 cwnd = 2 ACK = 3 cwnd = 3 ACK = 5 cwnd = 4 ACK = 7 cwnd = 5 SEQ = 1 SEQ = 2 SEQ = 3 SEQ = 4 SEQ = 5 SEQ = 6 SEQ = 7 SEQ = 8 SEQ = 9 SEQ = 1 in-flight = 2 in-flight = 3 in-flight = 3 ACK = 9 cwnd = 6 in-flight = 3 segment size ( bytes) B/s = segment transmission time Router Buffers Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 6

7 Error-Free Transmission 1 8 ACK SEQ cwnd 6 Latency = 2.77 Utilization = 34.2% goodput = 34.2% 4 2 Slow Start Collision Avoidance Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 7

8 Classic TCP-Reno Steady State in Collision Avoidance Self-Clocking in-flight cwnd in-flight in-flight - n n new ACKs n+1 new segments cwnw cwnd + 1 Sender cwnd Receiver time Router Buffers in-flight = cwnd = 2ACK in-flight = cwnd = 3 ACK = 3 SEQ = 1 SEQ = 2 SEQ = 3 SEQ = 4 SEQ = 5 B/s = cwnd segment size RTT Congestion RTT and cwnd B/s cwnd = 8 ACK = 6 Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 8

9 Congestion Longer RTT 1 8 ACK (no congestion) SEQ (no congestion) cwnd (no congestion) ACK SEQ cwnd 6 4 Latency = 3.7 Latency (lossless) = 2.77 Excess latency = 33.57% Utilization = 25.6% goodput = 25.6% Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 9

10 Steady State 1 8 ACK SEQ cwnd 6 Latency = 5.32 Latency (lossless) = 2.77 Excess latency = 92.6% Utilization = 2.8% goodput = 17.8% Packet 9 lost 3 dupacks Packet 29 lost 3 dupacks Packet 49 lost 3 dupacks Packet 69 lost 3 dupacks Packet 89 lost 3 dupacks Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 1

11 Classic TCP-Reno 1 Lost Packet Sender Receiver in-flight = cwnd = 4 dupack = 7 dupack = 7 dupack = 7 cwnd = 2 ACK = 7 SEQ = 7 SEQ = 8 SEQ = 9 SEQ = 1 SEQ = 7 X Resend 1 and Wait 1 RTT while buffers empty ACK = 11 RTT and cwnd B/s in-flight = cwnd = 3 SEQ = 11 SEQ = 12 SEQ = 13 cwnd Router Buffers time Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 11

12 1 Lost Packet ACK SEQ cwnd Latency = 4.46 Latency (lossless) = 2.77 Excess latency = 61.1% Utilization = 26.% goodput = 21.3% Packet 37 lost 3 dupacks Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 12

13 Classic TCP-Reno Serious Congestion Sender Receiver in-flight = cwnd = 4 ACK = 7 SEQ = 7 SEQ = 8 SEQ = 9 SEQ = 1 X X X X RTO No packets arrive no dupacks timeout B/s = cwnd segment size RTT Congestion RTT and cwnd B/s resend 1 in-flight = cwnd = 1 SEQ = 7 ACK = 7 cwnd time Router Buffers Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 13

14 1 Lost Packet Early 1 8 ACK SEQ cwnd 6 Latency = 6.87 Latency (lossless) = 2.77 Excess latency = 148.1% Utilization = 14.% goodput = 13.8% 4 2 Packet 3 lost Timeout Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 14

15 Lost Packet + Lost Retransmit 1 8 ACK SEQ cwnd 6 Latency = 6.89 Latency (lossless) = 2.77 Excess latency = % Utilization = 14.1% goodput = 13.8% 4 2 Packet 7 lost 3 dupacks Retransmit Packet Retransmit lost timeout Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 15

16 Classic TCP-Reno 1 Packet Error in-flight = cwnd = 4 dupack = 7 dupack = 7 dupack = 7 cwnd = 2 Sender ACK = 7 SEQ = 7 SEQ = 8 SEQ = 9 SEQ = 1 SEQ = 7 Receiver X Router Buffers No congestion Network supports large cwnd Resend 1 and Wait 1 RTT in-flight = cwnd = 3 ACK = 11 SEQ = 11 SEQ = 12 SEQ = 13 RTT and cwnd B/s Sender sees 3 dupacks Lowers cwnd Reno assumes lost packet congestion Low packet error on wired Ethernet BER 1-5 Moderate response Lose 1 RTT + time to recover cwnd cwnd time Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 16

17 Classic TCP-Reno Multiple Packet Errors in-flight = cwnd = 4 RTO Sender ACK = 7 SEQ = 7 SEQ = 8 SEQ = 9 SEQ = 1 No packets arrive no dupacks timeout Receiver X X X X Router Buffers No congestion Network supports large cwnd B/s = cwnd segment size RTT resend 1 in-flight = cwnd = 1 Errors RTT and cwnd B/s SEQ = 7 ACK = 7 Sender timeout Lowers cwnd Reno assumes multiple lost packets serious congestion P(k consecutive packet errors on Ethernet) (1-5 ) k Extreme response Lose RTO + time to recover cwnd in slow start cwnd time Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 17

18 The Trouble with Wireless 1 Variations in transmission medium Multiple correlated packet losses + bit errors TCP interprets as serious congestion timeouts + slow start Fading channels Refraction Reflection Absorption Multipath refraction reflection absorption medium EMI (electromagnetic interference) Other users Other radio equipment Other radiating equipment station station station station Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 18

19 The Trouble with Wireless 2 Mobility drops User moves between wireless domains Adds delays + buffers + dupacks + timeouts MSC MSC RNC-1 RNC-2 Clusters RNC-1 RNC Node-B Cells TCP request in cell 1 TCP response in cell 4 Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 19

20 The Trouble with Wireless 3 Link asymmetry Upstream channel slower than downstream channel Larger buffer longer buffer delay lower average B/s ACK compression Base Mobile ACKs delayed in upstream buffer RTT B/s All ACKs arrive together cwnd sharply Sender floods forward channel buffer Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 2

21 The Trouble with Wireless 4 MACA in WiFi Required to prevent hidden node problem RTS RTS CTS CTS A B C D E F DATA ACK MAC MACA overhead RTS+CTS+ACK MAC time TCP ACK time TCP WiFi ACK delay = 2 wired TCP ACK delay RTT Timeouts cwnd cwnd segment size B/s = RTT RTT and cwnd B/s Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 21

22 Typology of Approaches to Wireless TCP Split mode Mobile BSS Receiver MS / BSS specialized TCP BSS / server standard TCP Proposed in 199s End-to-end Refinements to TCP-Reno Better handling of non-congestion packet loss Example TCP TCP-Tahoe TCP-Reno Infrastructure-awareness Use IP and infrastructure layer information at TCP layer Example ECN provides IP router information to TCP Reactive congestion control React to congestion loss Example Reno cuts cwnd after 3 dupacks Proactive congestion control Predict available bandwidth from ACK rates Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 22

23 Multiple Losses in Reno Example 3 consecutive lost packets of 16 cwnd in-flight SEQ X ACK = 26 3 dupack = 26 ACK = dupack = 27 6 more dupack = ACK = dupack = 28 Timeout on cumack = 42 Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 23

24 New Reno RFC 6582 Maintain fast recovery until all in-flight ACKed cwnd in-flight X ACK = 26 3 dupack = 26 ACK = 27 ACK = 28 cumack = 31 Send next on partial ACK of resend ACK = 46 Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 24

25 TCP-NCL (Non-Congestive Loss) Heuristic Timeout could be error Retransmission timeout probably congestion Enhancement to Reno Treat first timeout as error Treat long timeout as congestion Two timeouts Retransmission Decision (RD) Cancel on ACK Retransmit on timeout Congestion Decision (CD) Cancel on ACK Congestion control on timeout ACK ACK Send Packet Start RD RD Resend Packet Start CD CD timeout timeout Start Congestion Control Update RTT C. Lai, K. Leung, V.O.K. Li, Enhancing Wireless TCP: A Serialized- Timer Approach, IEEE INFOCOM, 21. Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 25

26 TCP Vegas Standard Reno slow start cwnd = 1 cwnd cwnd + size of data ACKed Base expectation expected throughput = cwnd / RTT min Define thresholds α, β L. Brakmo and L. Peterson, TCP Vegas: End to End Congestion Avoidance on a Global Internet, IEEE JSAC, vol. 13, no. 8, Oct. 1995, pp Enhanced Reno congestion avoidance Measure RTT for distinguished (special) segment of some size actual throughput = size of ACK received / RTT diff = expected throughput actual throughput if (diff < α) then cwnd++ on each ACK if (diff > β) then cwnd-- on each ACK dupacks Retransmit on 1 dupack if RTT > RTT Vegas-timeout Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 26

27 TCP Modified Fast Retransmission (MFR) Reno Transmit cwnd packets Packet n lost k dupacks retransmit on k 3 dupacks Wait for ACK of n no dupacks if retransmit lost timeout Enhancement to Reno fast retransmit k dupacks k packets arrived k dupacks + 1 lost packet = k+1 packets accounted cwnd (k+1) unaccounted If (k > 3) cwnd = cwnd + (k 3) send more packets Cumulative ACK retransmitted packet + more arrived If (dupacks 3 > unaccounted) New packets arrived but retransmitted n lost Retransmit n again no timeout S. Prasanthi, S. Chung, "An Efficient Algorithm for the Performance of TCP over Multi-hop Wireless Mesh Networks", Seventh International Conference on Information Technology, pp , 21. Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 27

28 Modified Fast Retransmission Example Prevents timeout on repeated loss cwnd in-flight SEQ X X ACK = 21 3 dupack = 21 5 more dupack = 21 (8 of 1) 16 sent 5 ACK 8 dupack 1 lost = 2??? 3 dupack = 21 (late or new?) 6 more dupack = more dupacks > 2??? Resend 21* cumack = 39 * If more dupacks = 2 either: 21 lost again Late dupacks from Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 28

29 Modified Congestion Control (MCC) Reno congestion control ssthresh = cwnd Approach: Oops! Went to far. Cut back half. cwnd = cwnd / 2 Bandwidth Estimate (BWE) BWE = weighted average of bytes ACKed ΔRTT for bytes_to_ack packets ACKed bytes per packet BWE_s[k] = now last_ack_time BWE_s[k] + BWE_s[k-1] BWE[k] = (1 β ) +β BWE[k-1] 2 Modified congestion control 3 dupacks Timeout ssthresh = (BWE * ΔRTTmin) / segment size cwnd = ssthresh if (cwnd > ssthresh) cwnd = 1 ssthresh = max{(bwe*δrttmin) / segment size, 2} L. Yongmei, J. Zhigang, Z. Ximan, A New Protocol to Improve Wireless TCP Performance and Its Implementation, 5th International Conference on Wireless Communications, Networking and Mobile Computing, 29. Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 29

30 Proposed Solutions for WiFi Overhead Problem: MACA overhead Each TCP ACK RTS+CTS+ACK MAC RTS+CTS+ACK MAC time TCP ACK time Solution: reduce control traffic Increase TCP ACK delay Buffer more TCP ACKs in receiver ACK 4 TCP segments instead of 2 Reduce ACK traffic by 5% BWE not changed Bits ACKed and RTT RTS CTS DATA ACK MAC Problem: longer ACK delay lower utilization Sender waits longer for ACKs Solution: infrastructure awareness Sender writes cwnd in TCP option field Immediate ACK if unacked data in receiver ACK buffer cwnd Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 3

31 Freeze-TCP Handles mobility drops Handoff disconnect / reconnect Temporary fading by obstacles Mobile station monitors radio signal strength Predicts impending disconnections Freeze-TCP receiver proactively sets window = TCP sender enters persist mode On reconnect Receiver sends multiple ACKs of last received packet Prevents exponential back off of cwnd in sender Infrastructure awareness TCP layer exposed to details handled by lower layers Roaming / handoff at layers 2 / 3 Radio signal quality at layers 1 and 2 Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 31

32 TCP-Jersey Available Bandwidth Estimation (ABE) Similar to BWE expressed in segments Congestion Warning (CW) Requires explicit congestion notification (ECN) support in routers IP routers mark packets on congested link TCP receiver returns ECN warning to TCP sender CW without loss cwnd = ABE 3 dupacks Retransmit If (CW in dupack segment) cwnd = ABE Else no change to cwnd Timeout Retransmit If (CW in dupack segment) cwnd = 1 Else no change to cwnd Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 32

33 TCP-Jersey Results Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 33

34 TCP Westwood Modifies TCP sender Not dependent on negotiation with TCP receiver Not dependent on ECN support Estimates available bandwidth Counts ACKs and dupacks as successful traffic On packet loss set cwnd = available bandwidth Improves on Reno cwnd = cwnd / 2 Performance almost as good as Jersey Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 34

35 Westwood Packet Counting Reno enhancement Standard slow start Standard congestion avoidance Calculate bandwidth estimate (BWE) on each ACK BWE k d number of bytes ACKed at time tk t - t time of ACK time of previous ACK k = = k k-1 ( ) BWE = α BWE + 1 α k k k BWE + BWE 2 k k 1 Handling isolated error Out-of-order packets dupacks dupack some packets ARRIVED at receiver Count dupacks in bandwidth estimate On ACK of lost packet treat dupack as already counted Serious congestion Causes timeouts instead of dupacks Bandwidth sample Bandwidth estimate Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 35

36 Westwood Packet Counting Example Packet transmission times t t 1 t 2 t 3 t 6 Time ACK Arrived at Receiver Packets for BWE Counted dupack BWE t / (t 1 t ) t / (t 2 t 1 ) t / (t 3 t 2 ) t / (t 4 t 3 ) t / (t 5 t 4 ) t / (t 6 t 5 ) t / (t 7 t 6 ) ACK jumps to 1 from 5 but 3 ACKs (6, 7, 8) already counted as dupacks Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 36

37 Westwood ACK Counter (for BWE) newack = ACK prevack ; Packets ACKed by new ACK // if (newack = 1) do nothing No error condition report 1 ACK if (newack = ) count++ ; newack = 1 ; if (newack > 1) ACK "stuck" on old value (dupack) Increment dupack counter Count 1 dupack ACK advances if (count >= newack) count = count newack + 1 ; newack = 1 ; else if (count < newack) newack = newack - count ; count = ; Not all arrived packets ACKed Remove ACKed from dupack count Count as additional dupack All packets ACKed in order Report ACKed counted (new ACKs) Zero dupack counter prevack = ACK ; return(newack); Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 37

38 Westwood Scenario 1 No Errors Receive 1 2,3 4,5,6,7 8,9,1,11,12,13,14,15 ACK newack = newack = ACK prevack ; if (newack = ) count++ ; newack = 1 ; if (newack > 1) if (count >= newack) count = count newack + 1 ; newack = 1 ; else if (count < newack) newack = newack - count ; count = ; prevack = ACK ; return(newack); Total = Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 38

39 Westwood Scenario 2 1 Packet Out-of-Order Receive 1 2,3 5,4,6,7 8,9,1,11,12,13,14,15 ACK newack = ACK prevack ; if (newack = ) count++ ; newack = 1 ; if (newack > 1) 1 1 if (count >= newack) count = count newack + 1; newack = 1 ; else if (count < newack) newack = newack - count ; count = ; prevack = ACK ; return(newack); Total = Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 39

40 Westwood Scenario 3 1 Packet Very Out-of-Order Receive 1 2,3 5,6,7,4 8,9,1,11,12,13,14,15 ACK newack = ACK prevack ; if (newack = ) count++ ; newack = 1 ; if (newack > 1) if (count >= newack) count = count newack + 1 ; newack = 1 ; else if (count < newack) newack = newack - count ; count = ; prevack = ACK ; return(newack); Total = Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 4

41 Westwood Scenario 4 Variation of Scenario 3 Receive 1 2,3 5,6,7,4 8,9,1,11,12,13,14,15 ACK newack = ACK prevack ; if (newack = ) count++ ; newack = 1 ; if (newack > 1) if (count >= newack) count = count newack + 1 ; newack = 1 ; else if (count < newack) newack = newack - count ; count = ; prevack = ACK ; return(newack); Total = Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 41

42 Westwood Scenario 5 Another Variation of Scenario 3 Receive 1 2,3 5,6,7,4 8,9,1,11,12,13,14,15 ACK newack = ACK prevack ; if (newack = ) count++ ; newack = 1 ; if (newack > 1) if (count >= newack) count = count newack + 1 ; newack = 1 ; else if (count < newack) newack = newack - count ; count = ; prevack = ACK ; return(newack); Total = Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 42

43 Westwood Scenario 6 2 Packets Out-of-Order Receive 9,1,11,13,14,15,8,12 (after ACK = 8) ACK newack = ACK prevack ; 4 4 if (newack = ) count++ ; newack = 1 ; if (newack > 1) if (count >= newack) count = count newack + 1 ; newack = 1 ; 3 1 else if (count < newack) newack = newack - count ; count = ; 1 prevack = ACK ; return(newack); Total = Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 43

44 Westwind Congestion Control Reno slow start On (ACK && cwnd < ssthresh) cwnd cwnd + size of data ACKed Reno congestion avoidance On (ACK && cwnd > ssthresh) cwnd cwnd + 1 Modified fast recovery On n dupacks ssthresh = BWE * RTT_min / segment_size if (cwnd > ssthresh) cwnd = ssthresh Modified timeout ssthresh = BWE * RTT_min / segment_size if (ssthresh < 2) ssthresh = 2 cwnd = 1 Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 44

45 Westwood Performance Throughput Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 45

46 Westwood cwnd and ssthresh Westwood versus Reno 1 8 ACK SEQ cwnd Steady state cwnd (slide 1) Protocols and Networks Hadassah College Spring 218 Wireless Dr. Martin Land 46

TCP Westwood for Wireless

TCP Westwood for Wireless תוכן מבוא 1. רקע טכני בקרת עומס ב- TCP 2. TCP על קשר אלחוטי 3. שיפור תפוקה עם פרוטוקול TCP Westwood 4. סיכום 5. Seminar in Computer Networks and Distributed Systems Hadassah College