Mitigating Egregious ACK Delays in Cellular Data Networks by Eliminating TCP ACK Clocking

Transcription

1 Mitigating Egregious ACK Delays in Cellular Data Networks by Eliminating TCP ACK Clocking Wai Kay Leong, Yin Xu, Ben Leong, Zixiao Wang National University of Singapore

2 Asymmetry in Cellular Networks Congestion in Uplink Concurrent Upload RSFC [Xu et al. 2012] Other Users Uplink Downlink

3 Egregious ACK Delays TCP congestion control is ACKclocked Data ACK

4 Egregious ACK Delays TCP congestion control is ACKclocked Congested uplink can delay ACKs Downlink becomes idle Data ACK

5 Egregious ACK Delays TCP congestion control is ACKclocked Idle Downlink Congested uplink can delay ACKs Downlink becomes idle Under Utilization Data Reduced Throughput ACK

6 Solution: Eliminate ACK Clocking Idea: If we know the bandwidth, we can send at maximum rate. Send Data at X Mb/s cwnd Bandwidth: X Mb/s Data ACK Not dependent on ACK arrival

7 Challenge 1: Estimating Bandwidth Bandwidth Receive Rate Idea Use receiving rate as equivalent of available bandwidth Done Passively Condition To avoid modifications at the receiver Use TCP Timestamps Solution Enabled by default on Android and iphones

8 Estimating Receive Rate Sender Mobile Receiver TSval = t r0 TSval = t r1 TSval = t r0 TSval = t r1 Receive Rate ρ t r1 t r0 = Δt ΔACK/Δt=ρ

9 Challenges 1. Estimating Bandwidth 2. Timestamp Granularity too Coarse Cannot estimate with high accuracy 3. Bandwidth variation Have to keep updating estimation

10 Solution Self-oscillating Feedback Loop Estimate Receive Rate ρ Send Rate σ How to detect Send slower ( σ < ρ ) No Congestion congestion? Link Congested Send Faster ( σ > ρ )

11 Detect Congestion Idea: Monitor Queuing Delay Buffer t buff > 0 t buff = 0 How? TCP Timestamps Relative Difference between sender and receiver

12 Detecting Congestion Relative Delay (RD) t s1 x Queuing Delay (t buff ) Min Delay (RD min ) TSval = t r1 Sender RD RD min t buff x Mobile Receiver Congestion detected when t buff >T A TSval = t r1 RD = t r1 t s1

13 Summary of Algorithm 1. Initial Receive Rate Estimation Send 10 packets Estimate ρ using replies 3. Monitor Mode Probe network Details in paper 2. Buffer Management Mode TCP-RRE t buff < T a) Buffer Fill State Send Faster (σ >ρ) b) Buffer Drain State (Receiver-Rate Estimation) Significant changes in network Send Slower (σ <ρ) t buff > T

14 Parameters? How much faster or slower to send? What threshold T to use? When to switch to monitor state? See details in paper

15 ns-2 Evaluation Measured real networks to get simulation parameters

16 ns-2 Evaluation 1. Single Download with Slow Uplink 2. Single Download under Normal Conditions 3. Download with Concurrent Upload 4. Handling Network Fluctuation 5. TCP Friendliness

17 ns-2 Evaluation 1. Single Download with Slow Uplink 2. Single Download under Normal Conditions 3. Download with Concurrent Upload 4. Handling Network Fluctuation 5. TCP Friendliness

18 Download with Slow Uplink 1MB Data ACK Uplink Speed (kb/s) Downlink Speed (Mb/s) ns-2

19 Download with Slow Uplink Uplink speed decreases ns-2

20 Download under Normal Conditions ns-2

21 Download under Normal Conditions Packet Loss ns-2

22 Download under Normal Conditions ns-2

23 Download with Concurrent Upload Download 1MB Upload until download completion Uplink Speed (kb/s) Downlink Speed (Mb/s) ns-2

24 Download with Concurrent Upload 2.5X ns-2

25 Evaluation in Linux Several Places, different ISPs Multiple times Download 1MB with TCP-RRE CUBIC Upload until download completion CDF of all experiments at each place for each ISP

26 Evaluation in Linux

27 Evaluation in Linux +50%

28 Evaluation in Linux

29 Evaluation in Linux +25%

30 Conclusion Congested Uplink Delayed ACKs Poor Download Performance

31 Conclusion TCP-RRE ACK Clocking Rate Control with Feedback Loop Use TCP Timestamp Estimate Receive Rate Detect Congestion Improves TCP Uplink is Slow Uplink is Congested Keep the Delay Low Fair to Other TCP Flows

32 Thank You QUESTIONS

33 Handling Network Fluctuations CUBIC

34 Handling Network Fluctuations TCP-RRE

35 TCP Friendliness Run two RSFC uploads concurrently Calculate Jain fairness index: ( R + R ) /(2( R + R ))

36 TCP Friendliness