Investigating the Use of Synchronized Clocks in TCP Congestion Control

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1 Investigating the Use of Synchronized Clocks in TCP Congestion Control Michele Weigle Dissertation Defense May 14, 2003 Advisor: Kevin Jeffay

2 Research Question Can the use of exact timing information improve TCP congestion control? 2

3 Claim synchronized clocks exact timing information early congestion detection less packet loss and shorter queues better overall network performance 3

4 Outline Background Related Work Thesis Statement Sync-TCP Evaluation Conclusions Future Work 4

5 Background Queuing Router queues are FIFO and finite the longer the queue, the longer a packet at the end of the queue is delayed if queue is full, incoming packets are dropped X Most queues are drop-tail incoming packets are only dropped when the queue is full 5

6 Background Congestion Sustained period where the incoming rate is greater than the service rate Leads to increased queuing delays Leads to packet loss leads to increased latency for TCP flows leads to low throughput 6

7 Background TCP Data Transfer sender receiver RTT data 1 ACK 2 OTT (data) OTT (ACK) congestion window size (cwnd) = 1 data 2 throughput = cwnd / RTT time time 7

8 Background TCP Congestion Window sender receiver RTT data 1 data 2 data 3 ACK 2 ACK 3 ACK 4 data 4 data 5 data 6 cwnd = 3 throughput = cwnd / RTT time time 8

9 Background TCP Congestion Control Available network bandwidth is unknown TCP probes the network by increasing the congestion window when ACKs return TCP backs off by reducing the congestion window when loss is detected 9

10 Background TCP Reno Loss Detection 3 Duplicate ACKs reduce congestion window by 50% Retransmission Timeout reduce congestion window to 1 packet congestion window throughput = cwnd / RTT x duplicate ACKs x time timeout 10

11 Background TCP Reno Data Recovery sender ACK 2 data 1 data 2 data 3 data 4 data 5 X ACK 2 ACK 2 ACK 2 data 6 receiver data 2 time time 11

12 The Problem TCP Congestion Control Overflows queues in search for more resources Uses packet loss as its only indicator of congestion relies on a binary signal of congestion 12

13 The Problem Congestion Control congestion window x duplicate ACKs time x timeout TCP Reno: React to packet loss reduce sending rate only when packets are lost perform congestion control only when it is time to retransmit lost packets congestion window Goal: React to congestion early and avoid losses congestion occurs before packets are lost decouple congestion control and retransmission time 13

14 Related Work Congestion Control End-to-End TCP Reno is the problem Internet adaptation router Router-based drop-tail queues are the problem active queue management (AQM) router adaptation router Internet adaptation router 14

15 Related Work Congestion Control End-to-End Delay-based congestion control [R. Jain, 1989] TCP Vegas [Brakmo, O Malley, Peterson, 1994] TCP Santa Cruz [Parsa, Garcia-Luna-Aceves, 1999] TCP Westwood [Mascolo, Casetti, Gerla, Sanadidi, Wang, 2001] TCP Peach [Akyildiz, Morabito, Palazzo, 2001] Binomial algorithms [Bansal, Balakrishan, 2001] Router-based DECbit [Ramakrishnan, R. Jain, 1990] Random Early Detection (RED) [Floyd, Jacobson, 1993] Explicit Congestion Notification (ECN) [Floyd, 1994] Adaptive RED [Floyd, Gummadi, Shenker, 2001] 15

16 Thesis Statement Precise knowledge of one-way transit times can be used to improve the performance of TCP congestion control. 16

17 Thesis Statement Precise knowledge of one-way transit times can be used to improve the performance of TCP congestion control. network-level metrics: packet loss and average queue sizes at congested routers application-level metrics: HTTP response times and goodput per HTTP response 17

18 Thesis Statement Precise knowledge of one-way transit times can be used to improve the performance of TCP congestion control. provide lower packet loss and lower queue sizes than TCP Reno provide lower HTTP response time and higher goodput per HTTP response than TCP Reno 18

19 My Approach 1. Exchange exact timing information 2. Detect congestion 3. React to congestion 4. Sync-TCP congestion control 5. Evaluate Sync-TCP vs. TCP Reno 19

20 Sync-TCP Synchronized Clocks Allow measurement of OTT Methods of synchronization Global Positioning System (GPS) Network Time Protocol (NTP) Internet 20

21 Sync-TCP TCP Header Option New option in the TCP header 14 bytes type OTT (ms) timestamp echo reply length head len 32 bits source port # dest port # sequence number acknowledgment number not used checksum UAP R S F rcvr window size ptr urgent data options (variable length) application data (variable length) 21

22 Sync-TCP sender Example [OTT, timestamp, echo reply] [-1, 1, -1] [1, 3, 1] [1, 5, 3] [2, 8, 5] receiver Sender s Calculations time data received = time data sent (echo reply) + OTT time ACK delayed = time ACK sent (timestamp) - time data received queuing delay = OTT - minimum OTT time time 22

23 Sync-TCP Congestion Detection 50% of maximum-observed queuing delay (queuing delay = OTT minimum-observed OTT) 50% of minimum-observed OTT Average queuing delay Trend analysis of queuing delays Trend analysis of the average queuing delay 23

24 Sync-TCP Trend Analysis of Average Queuing Delay Trend analysis for available bandwidth estimation adapted from [Jain and Dovrolis, 2002] Operation: compute 9 average queuing delay samples split into 3 groups of 3 samples each compute median, m i, of each group trend is relationship of m 1, m 2, m 3 24

25 Sync-TCP Trend Analysis of Average Queuing Delay Every arriving ACK, compute smoothed average queuing delay from OTT Compute trend of average queuing delay after first 9 ACKs afterwards, every 3 ACKs Calculate the average queuing delay as a percentage of the maximum-observed queuing delay divide into 25% increments 25

26 Sync-TCP 100 Queuing Delay at Router queuing delay at router 80 queuing delay (ms) time (s) 26

27 queuing delay (ms) Sync-TCP Trend Analysis of Average Queuing Delay queuing delay at router average computed queuing delay increasing trend decreasing trend max 75% 50% 25% time (s) 0% 27

28 Sync-TCP Congestion Reaction Decrease congestion window by 50% upon congestion notification same reaction as TCP Reno to packet loss Increase and decrease congestion window according to congestion signal intended to be used with trend analysis of average queuing delay congestion detection operates the same as TCP Reno until 9 ACKs have been received 28

29 average queuing delay Sync-TCP Congestion Window Adjustment increasing trend decrease 50% decrease 25% decrease 10% increase 1 packet per RTT time decreasing trend no change increase 25% per RTT increase 50% per RTT increase 10% per RTT max 75% 50% 25% 0% 29

30 Sync-TCP Congestion Control Congestion Detection Trend analysis of smoothed average queuing delay 50% of maximum queuing delay 50% of minimum OTT Smoothed average queuing delay Trend analysis of queuing delays Congestion Reaction Increase and decrease congestion window according to congestion signal Decrease congestion window by 50% upon congestion notification 30

31 Evaluation Experiment Plan NS-2 network simulator assume synchronized clocks FTP bulk-transfer traffic examine the steady-state operation of the mechanisms HTTP traffic integrate traffic model developed at Bell Labs into NS-2 main parameter is average number of HTTP requests per second calibrate HTTP request rate to desired load level 31

32 Evaluation HTTP Simulation Environment Sync-TCP and TCP Reno flows do not compete Two-way traffic measure performance in one direction only new HTTP requests generated per second 45-2,500 HTTP connections active simultaneously 250,000 HTTP requestresponse pairs completed web servers web clients 10 Mbps request response web clients web servers 32

33 Evaluation HTTP Experiment Space Sync-TCP congestion control mechanism 50% max queuing delay detection and reduce by 50% reaction trend analysis of average queuing delay detection and adjust according to signal reaction TCP for comparison TCP Reno, TCP SACK Queuing method for comparison drop-tail, Adaptive RED, Adaptive RED with ECN End-to-end load (% of link capacity) 50%, 60%, 70%, 80%, 85%, 90%, 95%, 100%, 105% Number of congested links 1, 2 (75% total load, 90% total load, 105% total load) 33

34 Evaluation Evaluating HTTP Performance Network-level Metrics packet loss at bottleneck router queue size at bottleneck router Application-level Metrics goodput per HTTP response bytes received per second at web client HTTP response times time between sending the request and receiving the entire response 34

35 Evaluation 8 6 Average Packet Loss at Bottleneck TCP Reno Sync-TCP 400 K packet loss % K 0 8 K 0 21 K K 5 K 100 K 25 K 180 K 90 K 275 K 50% 60% 70% 80% 85% 90% 95% offered load 35

36 Evaluation Average Queue Size at Bottleneck 80 TCP Reno Sync-TCP queue size (packets) % 60% 70% 80% 85% 90% 95% offered load 36

37 Evaluation Average Goodput per Response 160 TCP Reno Sync-TCP goodput (kbps) % 60% 70% 80% 85% 90% 95% offered load 37

38 Response Time CDF Example 100 cumulative probability ~75% of the responses completed in 400 ms or less HTTP response time (ms) 38

39 Response Time CDF 50% Load 100 cumulative probability No large difference between uncongested and congested HTTP response time (ms) 39

40 Response Time CDF 70% Load 100 cumulative probability Sync-TCP performs slightly better than TCP Reno HTTP response time (ms) 40

41 Response Time CDF 80% Load 100 cumulative probability Sync-TCP performs better than both TCP Reno and AQM HTTP response time (ms) 41

42 Response Time CDF 85% Load 100 cumulative probability Sync-TCP performs better than both TCP Reno and AQM HTTP response time (ms) 42

43 Evaluation Early Congestion Detection Sync-TCP early congestion detection only operates after 9 ACKs have been received HTTP responses > 25 KB Only 7-8% of HTTP responses > 25 KB HTTP responses < 25 KB do not use Sync- TCP early congestion detection use TCP Reno 43

44 Evaluation congestion window (packets) 85% Load, 48 MB Response TCP Reno (17 ms base RTT) x packet drop (952) Sync-TCP (47 ms base RTT) x packet drop (190) time (s)

45 Conclusions Sync-TCP performs better than TCP Reno packet loss average queue size goodput per HTTP response HTTP response time Sync-TCP has comparable performance to best TCP and AQM combination Limitations of delay-based congestion control may not compete well with TCP Reno on same network with many congested links, decrease in one queue could mask increase in another queue 45

46 Summary synchronized clocks one-way transit times early congestion detection Taking advantage of synchronized clocks in TCP can result in better network performance. less packet loss and shorter queues better overall network performance 46

47 My Contributions Method for measuring a flow's OTT and returning this exact timing information to the sender Comparison of several methods for using OTTs to detect congestion Sync-TCP: a family of end-to-end congestion control mechanisms based on using OTTs for congestion detection 47

48 Supporting Work Study of standards-track TCP congestion control and error recovery mechanisms in the context of HTTP traffic Weigle, Jeffay, and Smith, Quantifying the Effects of Recent Protocol Improvements to Standards-Track TCP, in submission. Additions to NS-2 integrated a state-of-the-art random number generator integrated Bell Labs HTTP traffic model developed a module for delaying and dropping packets on a per-flow basis according to a given distribution Heuristics for determining appropriate run length for HTTP simulations 48

49 Future Work Further Analysis accuracy of clock synchronization multiple congested links Sync-TCP with router support Extensions to Sync-TCP improve congestion detection and reaction ACK compression ACK congestion control improve fairness Uses for synchronized clocks in TCP statistics for time-critical applications wireless devices 49

50 Thank You Committee Members Kevin Jeffay Don Smith Ketan Mayer-Patel Sanjoy Baruah Bert Dempsey Jasleen Kaur UNC Department of Computer Science My parents, Mike & Jean Clark My husband, Chris 50

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