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A Smart TCP Acknowledgment Approach for Multihop Wireless Networks

Ruy de OliveiraDepartment of Computer Science, CEFET-MT, Brazil

Torsten Braun, Member, IEEEInstitute of Computer Science and Applied Mathematics, University of Bern, Switzerland

IEEE Transactions on Mobile Computing, Vol. 6, No.2, February 2007PDF created with pdfFactory trial version www.pdffactory.com

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Outline

n Introductionn Dynamic Adaptive Acknowledgment

¨ Design Issues¨ Algorithm¨ Packet Loss Handling¨ An Improved Delaying Window Strategy for High Loss Scenarios

n Performance Evaluationsn Conclusions

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Introduction

n TCP has been successful due to its robustness and providing reliability on an end-to-end basis ¨ To motivate the extension of this protocol to wireless networks

n These networks pose some critical challenges to TCP¨ The level of bit error rate (BER) is not negligible¨ High mobility

n The focus of this paper is the interaction between TCP and the MAC layer




Introduction (cont.)

n To present a strategy for decreasing medium contention as much as possible

n The receiver may combine up to four ACK packet when the wireless channel is in good condition¨ Less for lossy channels

n The simple changes considerably diminish the number of transmissions over the wireless medium¨ The overall energy consumption is significantly reduced




Dynamic Adaptive Acknowledgment- Design Issues

n If the receiver acknowledges every incoming data packet¨ The probability of collisions between data and ACK packets

increases considerably

n The receiver contend for the medium by RTS/CTS control frames, the overall overhead at the MAC layer is not negligible

Time medium is busy for data and ACK transmissionin a 1-hop scenario (in msec)

8.9% → 2.2%




Dynamic Adaptive Acknowledgment (cont.) - Algorithm

n There are two changes on the regular TCP sender¨ The number of duplicate ACKs is decreased from three to two¨ The retransmission timeout (RTO) interval is increased fivefold

n The delay management is performed through a delaying window (dwin) at the receiver¨ dwin is initialized to one¨ Increases gradually for each received data packet

n Until it reaches four

n The sender cwnd limit is also set to four





n The receiver keeps a timer and keeps track of the sequence numbers of incoming data packets

n The receiver immediately sends an ACK to the sender and reduces dwin to the size of two packet¨ It gets a out-of-order packet¨ Filling a gap in its buffer¨ When its timer expires





n Upon arrival of a given data packet pi+1, it calculates the smoothed packet interarrival interval¨

n ： The last calculated valuen ： The packet interarrival intervaln α is the interarrival smoothing factor with 0 < α < 1

TCP-DAA receiver mechanisms

(0.75)





n Timeout interval ¨ k is a timeout tolerance factor with k > 0

n The dwin growth is governed by

¨ The startup speed factor μ, with 0 < μ < 1¨ The startup phase is over

n When it first reaches its maximum valuen maxdwin becomes true

(0.2)

(0.3)




Dynamic Adaptive Acknowledgment (cont.) - Packet Loss Handling

Delayed acknowledgment strategies

TCP-DAA LDA

cwnd = 10

TCP-DAA provides more stability regarding the number of delayed ACKs




Dynamic Adaptive Acknowledgment (cont.) - An Improved Delaying Window Strategy for High

Loss Scenarios

n If the channel is facing constant losses¨ To reduce the dwin to one¨ The dwin should be enlarged by less than one for every data

packet received

An alternative delaying window strategy for robustness against lossesdwin is reduced fully to one, and then increased slowly by steps defined by the μ’ parameter




Dynamic Adaptive Acknowledgment (cont.)- An Improved Delaying Window Strategy for High

Loss Scenariosn The receiver should provide enough ACKs to its corresponding

sender in order to prevent timeout

n It also to trigger the sender cwnd growth properly until its limit¨ Assume the sender is timeout

n Slow start threshold is set to two and cwnd is reset to onen To increase its cwnd by one when the next ACK arrives n It enters the congestion avoidance phasen The cwnd increase for the ith received ACK is given by

¨ cwndi-1： The pervious value of cwnd




Dynamic Adaptive Acknowledgment (cont.)- An Improved Delaying Window Strategy for High

Loss Scenarios

n

¨ cwnd0 is the value to which cwnd is set just after a slowdown n cwnd0 = 1

¨ cwndi is the value of cwnd at the ith increase step¨ W = 4, n = 7

n m + m/2 + m/3 = 7, m = 3.8n μ’ = 1/m = 0.26

n




Performance Evaluations

n The evaluation of TCP-DAA considering many aspects¨ Throughput, energy consumption and friendliness

n To compare with the main TCP (NewReno) and Large Delayed ACK (LDA)¨ Also compare with other TCP versions

n To simulate the other TCP versions with two improvements¨ Delayed ACK (DA)¨ Window limit (WL)




Performance Evaluations (cont.)

n There are two scenarios¨ Chain topology¨ Grid topology

Grid topology

Chain topology

General Simulation Parameters

200m

200m




Performance Evaluations (cont.) - Performance in the Chain Topology

Throughput

n These results are obtained by taking the average of five runs

TCP-DAA

TCP+DA+WL

SACK+DA+WL

LDA

TCP

TCP+DAVEGAS+DA+WL

Aggregate throughput in the chain topology

TCP-DAA vs. TCP：↑40%

TCP-DAA vs. LDA：↑17%

1, 2, 3, 4, 6-hop: The results are similar





Fairness

n In order to measure the ability of a mechanism in allowing a fair distribution of bandwidth

n The well-known fairness index¨ xi：the normalize throughput of i-th flow¨ n：the number of connections

Fairness for 10 Flows Sharing the Medium (Chain Topology)





Energy Efficiency

n To use the simple energy model implemented in the ns2¨ E(ni) = Etx(ni) + Erx(ni) + (N – 1) * E0(ni)

n ni：Node nin E： The total amount of energyn Etx： The energy spent by transmissionn Erx： The energy spent by receptionn E0： The energy spent by overhearing

n ，where e is the energy / bit ratio¨ pkt： The amout of packet transmitted by the sender¨ e_spent： The energy in joules spent by the sending node





Energy Efficiency

TCP

TCP+DA+WLLDA

TCP-DAA

Energy consumption at the TCP sender

Effect of number of hops Effect of packet size

TCP-DAA

LDA

TCP+DA TCP

256 512

1460

Number of hops：4TCP-DAA vs. TCP：↓26%




Performance Evaluations (cont.) - Performance in the Grid Topology

Throughput

Fairness Fairness in the Grid Topology

Aggregate throughput in the grid topology with cross traffichorizontal horizontal + vertical

TCP+DA+WLTCP-DAA




Performance Evaluations (cont.) - Optimization for Highly Noisy Environments

n RTO was decreased from fivefold to twofoldn μ’ = 0.2 and μ’ = 0.3 tend to provide highest performance

Optimal μ’ parameter for TCP-DAAp

μ’ = 0.2μ’ = 0.3




Performance Evaluations (cont.) - Optimization for Highly Noisy Environments

TCP-DAA

TCP-DAApTCP+DA+WL

TCP-DAA

TCP-DAAp

Comparison between the two TCP-DAA versions TCP-DAAp provides robustness for highly noisy scenarios

Under moderate condition

(As effectively as TCP+DA)

To need an adaptive receiver mechanism to switch between DAA and DAAp




Performance Evaluations (cont.) - TCP Friendliness

TCP-DAA

TCP+DA+WLTCP+DA+WL

TCP-DAA

Reference681

469

TCP-DAA friendliness Aggregate throughput for two distinct TCP versions in parallel under no hidden node problem effects

To need a sort of pacing for controlling its sending rate in mixed scenarios involving non-TCP-DAA flows




Conclusions

n The strategy aims to minimize collisions resulting from mutual interference between data and ACK packets¨ Transmitting as few ACKs as possible

n The mechanism is tailored to networks comprising at most 10 hops and facing moderate bit error rates

n The simulation show that it can outperform not only conventionalTCP but also similar techniques




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