CS 525M Mobile and Ubiquitous Computing Seminar TCP Westwood - - PowerPoint PPT Presentation

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CS 525M Mobile and Ubiquitous Computing Seminar TCP Westwood - - PowerPoint PPT Presentation

Jul 05, 2023 127 likes •412 views

CS 525M Mobile and Ubiquitous Computing Seminar TCP Westwood Written by S. Mascolo, C. Casetti, M. Gerla, M. Sanadidi and R. Wang Presented by Choong-Soo Lee Introduction TCP suffers performance degradation in wireless environment

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SLIDE 1

CS 525M – Mobile and Ubiquitous Computing Seminar

TCP Westwood

Written by S. Mascolo, C. Casetti,

  • M. Gerla, M. Sanadidi and R. Wang

Presented by Choong-Soo Lee

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SLIDE 2

Introduction

  • TCP suffers performance degradation in wireless

environment – TCP uses dropped packets as a congestion signal but packets can be lost due to other reasons in wireless environment. – End-to-end, link layer and split connect approaches

  • Local recovery is more effective than end-to-end

approach in handling wireless losses but end-to- end requires minimum support from routers and base stations

  • Authors propose TCP Westwood, an end-to-end

solution to handle wireless losses

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SLIDE 3

Outline

  • Introduction
  • TCP Westwood
  • Evaluation
  • Conclusion and Future Work
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SLIDE 4

ACK-based measurement

  • Source performs an end-to-end estimate of

the available bandwidth

  • Enables faster recovery
  • When an ACK is received, it uses the

corresponding data packet size to compute the bandwidth estimate

  • In case of a DUPACK, an average segment

size is used instead.

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SLIDE 5

Filtering ACK Reception Rate

  • Sample of Bandwidth Estimate
  • We need a low-pass filter to average sampled

measurements and to obtain the low frequency components of the samples.

1 1 1 1 1

2 1 ˆ ˆ 2 2 1 1

k k k k k k k k k k

t t b b b b t t t t τ τ τ

− − − − −

− − + = + + + − −

1 k k k k

d b t t − = −

1/ : cut-off frequency τ

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SLIDE 6

Filtering ACK Reception Rate

  • An Example

– Consider constant time interval where a = 0.9

  • Real ACKs are irregular and we need to ensure

that we have a sample within a certain interval. – Use of a virtual sample of 0

( ) ( )

1 1

1 ˆ ˆ 2

k k k k

a b a b b b

− −

− = ⋅ + +

2 1 ˆ ˆ 2 1

h k h k

m b b m

+

−   =   +  

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SLIDE 7

Delayed and Cumulative ACKs

  • Delayed ACK

– Send ACK every other in-sequence segment

  • Cumulative ACK

– Acknowledges all segments prior to its sequence number

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SLIDE 8

Delayed and Cumulative ACKs

cumul_ack =current_ack_seqno – last_ack_seqno; If (cumul_ack = 0) accounted_for++; cumul_ack = 1; endif if (cumul_ack > 1) if (accounted_for >= cumul_ack) accounted_for = accounted_for – cumul_ack; cumul_ack = 1; else if (accounted_for < cumul_ack) cumul_ack = cumul_ack – accounted_for; accounted_for = 0; endif endif last_ack_seqno = current_ack_seqno; acked = cumul_ack; return acked;

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SLIDE 9

n DUPACKs and Timeout

  • n duplicate ACKs and coarse timeout imply that there is

congestion in the network – ssthresh set to the available pipe size

if (n DUPAcKs are received) ssthresh = (BWE * RTTmin) / seg_size; if (cwin > ssthresh) cwin = ssthresh; endif endif If (coarse timeout expires) ssthresh = (BWE * RTTmin) / seg_size; if (ssthresh < 2) ssthresh = 2; endif cwin = 1; endif

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SLIDE 10

Outline

  • Introduction
  • TCP Westwood
  • Evaluation
  • Conclusion and Future Work
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SLIDE 11

Bandwidth Estimation

  • TCP Westwood with concurrent UDP Traffic
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SLIDE 12

Fairness

  • Fairness: flows using the same protocol

should get equal share of bandwidth

without RED with RED

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SLIDE 13

Friendliness (Simulation)

  • Friendliness: protocols using different protocols

should get equal share of bandwidth

  • Simulation setup

– 2 Mbps bottleneck bandwidth with 100ms round-trip time

0.1078 0.0913 10 10 0.0994

  • 20
  • 0.0992

20 TCP W estw ood TCP Reno TCP W estw ood TCP Reno Achieved Throughput ( Mbps) Num ber of Flow s

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SLIDE 14

Friendliness (Testbed)

  • Testbed implementation using Linux
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Performance (Setup)

  • Independent loss model in ground radio

environment

Source Destination Base Station

10 Mbps 45ms 2 Mbps 0.01ms

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SLIDE 16

Performance

  • Throughput vs. error rate
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SLIDE 17

Performance

  • Throughput vs. one-way propagation delay
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SLIDE 18

Performance

  • Throughput vs. link capacity
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SLIDE 19

Performance

  • cwin and ssthresh

TCP Westwood TCP Reno

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Performance

  • Burst error models

– Two state Markov model – It is used to model fading and blackout.

Good Bad ?gb ?bg

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SLIDE 21

Performance

  • Fading

– Good State (8 sec, 0.001%) Bad State (4 sec, 0 to 30%) – Throughput vs. error rate

Bad State Two-State Model

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SLIDE 22

Performance

  • Blackout

– Good state (4sec) – Bad state (0 to 0.5 sec) (blackout) – Throughput vs. average duration

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SLIDE 23

Performance

  • Throughput vs. link capacity of the Satellite

link

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SLIDE 24

Outline

  • Introduction
  • TCP Westwood
  • Evaluation
  • Conclusion and Future Work
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SLIDE 25

Conclusion

  • The authors proposed a new version of

TCP, TCP Westwood. – TCP Westwood offers faster recovery without over-shrinking cwin – TCP Westwood’s congestion control relies on both packet losses and bandwidth estimation – TCP Westwood improves performance

  • ver wide range of wireless/wired

scenarios

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SLIDE 26

Future Work

  • Include TCP NewReno feature for

recovering from multiple losses in the same window

  • Backward path could be the bottleneck

– Need to define fairness between DATA and ACK streams

  • More comparison between TCP Westwood

and link-layer techniques

  • Further refine bandwidth estimation and

filter method for better friendliness

  • Develop control theoretical model to study

TCP Westwood’s stability

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SLIDE 27

Questions

?