Alleviating the effects of mobility on TCP Performance Signal Strength based Link Management
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Alleviating the effects of mobility on TCP PerformanceSignal Strength based Link ManagementFabius Klemm*, Srikanth Krishnamurthy+
and Satish Tripathi+,* EPFL Lasaunne ,+ University of California, Riverside
Paper Presented by Dr.Nitin Vaidya, UIUC
Presentation at UCLA on June 6th , 2002
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Outline
• Motivation for Research
•Using Lower Layer Support to improve TCP performance
• Link Failures – True and False
• Signal Strength based methods to help improve TCP goodput
• Preliminary experiments and results
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Motivation
• TCP is unable to differentiate between true and false link failures – former due to mobility, latter due to congestion.
• Implement link layer mechanisms that can help:
• Anticipate real link failures by signal strength measurements – preemptively initiate route discovery.
• Reactively increase power level for transmission upon the detection of a real link failure to salvage TCP packets in transit.
• Requires mechanisms for differentiating between true and false link failures.
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Source DestinationLink breaks
RERRRERR
Background -- AODV
•Ad hoc On-Demand Distance-Vector
• Route discovered by queries. RERR message sent upon discovery of a link failure.
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RTS
CTS
Revisiting the IEEE 802.11 MAC protocol
•RTS – CTS – DATA – ACK
•Solves the hidden and exposed terminal problem in most cases.
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•Neighbor within reach•Mac Layer cannot establish RTS/CTS Handshake•Mac Layer reports link break to upper layers
False Link Failure Reports
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How come?
1 2 3 4 5DataRTS
• Transmission Range: 250 m• Interference Range: 550 m
4 is sending Data to 51 is sends an RTS to 2
2 does not send a CTS because it senses the transmission of 4
Node 1 gives up after seven times
False link layer report
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Objective
• In this preliminary work, we consider sparse scenarios and use a rather naïve approach to differentiating between true and false link failures.
• In reality, more sophisticated techniques might be needed.
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300 x 1500 meters
• Distance between TCP source and sink:– about 1530 m or 8.8 hops in average
Simulation Scenario
•50 mobile nodes + 2 static nodes
•1 TCP connection
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• Reduce packet losses due to mobility (correct link breaks)• Reduce packet losses due to false link failure reports
Problems to be solved
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Low Mobility: False link failures dominateHigh Mobility: Correct link failures dominate
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Maximum speed in m/s
Mobility-inducedlink breakages
False link failures
Reasons for packet loss
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Neighbor ID Timestamp 1 Distance 1 Timestamp 2 Distance 2
3 4.200 200 4.205 201
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• Node computes distance from signal strength – simple model is assumed wherein the attenuation is inversely proportional to the square of the distance.
• The time stamps correspond to the last two instances when the node heard the neighbor.
Each node maintains a Mac layer neighbor table:
Tackling False Link Failures
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Tackling False Link Failures
Persistent Mac
–A Node sends RTS packets more than seven times if neighbor is likely to be within transmission range.
– Simple naïve approach.
– Seems to work in the sparse scenarios considered.
– More sophistication may be needed in dense scenarios.
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Unchanged Mac
Persistent Mac
Persistent Mac – Packet Loss
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Mobility-inducedlink breakages
False link failures
Persistent Mac – Link Breakages
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Persistent Mac
Persistent Mac - Goodput
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Persistent Mac
Persistent Mac – TCP Retransmissions
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• Salvage in transit packets:
Source DestinationLink breaks
Lost packets due to Mobility
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Salvage Packets
•Two Approaches:
1.Proactive: Predict link breakage and stimulate the source to preemptively initiate a route discovery.
2.Reactive: Re-establish a broken link with a temporary higher transmission power level.
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T1 T2current
timefuture time
distance
• Mac layer informs routing layer when next hop is almost out of range
Mac Layer: Proactive
•Nodes use neighbor table to predict node movement in the future:
–Simple prediction: Assume linear node movement
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Mac Layer: Reactive
• Node raises transmission power temporarily if it cannot establish an RTS/CTS handshake
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1 2
RTS
RTS – Frame contains power value
CTS
Node 2 sends CTS with the same power
Same for Data and ACK
Node 2 moves out of range of Node 1
Reactive Mac
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•Node 3 is receiving Data from Node 4
Node 2 does not know about the data transferThe high power CTS collides with the Data at Node 3
•Node 1 establishes a high power link
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RTS
CTS
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Data 4
Problems!
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Source Destination
Stop sending!
Stop sending!
Salvaging Packets
•Routing layer informs source to stop sending
•But: Intermediate nodes keep forwarding packets
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Salvaging Packets
•Routing Layer
–Three route states:
•Down: no route
•Up: route ok, answer route requests
•Going Down: “weak route”, use only to salvage packets, do not answer route requests! NEW!
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Unchanged Mac
Proactive LM
Reactive LM
Combined
Results – Packet Loss
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Proactive LM
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Results - Goodput
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Maximum speed in m/s
TC
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Unchanged Mac
Proactive LM
Reactive LM
Combined
Results – Retransmissions per transmitted packet
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Conclusions & Future Work
• The methods proposed seem to improve TCP performance by as much as 40 % in the scenarios considered.
• The reactive scheme might cause problems in highly congested scenarios – especially when the network is dense.
• More sophisticated methods may be needed to clearly differentiate between link failures due to mobility and congestion.
• A node might need to more intelligently decide upon when to increase its transmission power level.
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Thank You