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The Impact of Multihop Wireless Channel on TCP Throughput and Loss
Presented byScott McLaren
Zhenghua Fu, Petros Zerfos, Haiyun Luo, Songwu Lu, Lixia Zhang, Mario Gerla (UCLA), INFOCOM 2003, San Francisco, Mar. 2003.
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Overview
Introduction Background Throughput in Multihop Wireless Networks Loss Behavior Improving Performance Conclusions
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Introduction
Improve channel utilization by spatial channel reuse
A TCP window size W* exists at which throughput is maximized by achieving best spatial reuse
Increasing the window size past W* will reduce throughput
Standard TCP typically grows its average window much larger than W*
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Techniques to improve efficiency
Link-REDTune the wireless link’s drop probability
Adaptive link-layer pacing scheme Increase the spatial reuse of the channel
Allow TCP to operate in the contention avoidance region
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802.11
RTS/CTS messagesNodes hearing this handshake defer
transmission until current transmission is finished
Data is dropped if no CTS is received after 7 RTS retries
Data is also dropped if 4 transmissions are sent without an receiving an ACK
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Hidden Terminals
A hidden terminal is a node in the receiver’s neighborhood, that can’t detect sender and may disrupt transmissions
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Nodes are 200m apart Transmission range is 250m Carrier sensing and interference range is 550m D is a hidden terminal of A B
Cannot hear CTS ( > 250m ) Cannot hear data from A, A is outside of D’s carrier sensing
range D can transmit to E
Causes collision at B, since D is within 550m interference range for B
Contention loss at B
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Chain Topology
Best throughput when window size is h/4Assuming ideal MAC protocol and equal packet
sizes Max concurrent senders is h/4, where max
spatial reuse is achieved TCP window size < h/4 under utilization TCP window size > h/4 reduced
throughput
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Cross Topology
2 TCP flows Best window W* = 2,
measured window = 12 20% throughput
reduction
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Grid Topology
4, 8, and 12 TCP flows
½ of flows in each direction
Measured TCP windows are larger than max achievable throughput
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Results
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TCP Loss Behavior
Using 8-hop chain, all 165 TCP drops out of 12349 transmissions were due to link drops
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TCP Loss Behavior
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Corollaries
m – number of backlogged nodes B* – the max number of nodes that can transmit their DATA packets
concurrently without collision C* – denotes the max number of nodes that can initiate RTS messages Corollary 4.1
m < B* Pl ≈ 0
Corollary 4.2 m > B* Pl increases as m increases
Corollary 4.3 m > C* Pl remains constant
Throughput reduction due to Wavg >> W*, Pl > 0, Link contention > 0 reducing spatial reuse
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Improving TCP Performance
Distributed Link RED (LRED) Adaptive Pacing
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LRED
Easy way is to improve performance by reducing buffer size, but problems with bursty traffic
LRED exploits dropping in 802.11 MAC RED provides a linearly increasing drop curve as
queue exceeds a min size LRED provides a linearly increasing drop curve
as link drop probability exceeds a min size
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LRED
Link layer maintains average number of retries
Next packet is dropped/marked with probability based on average number
If average number of retries is small, packets are not dropped/marked
When retries increase, the dropped/marked probability is calculated
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Adaptive Pacing
Improve spatial channel reuse by balancing traffic among nodes
Exposed receiver problem Let a node backoff an additional packet
transmission time when necessary
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Adaptive Pacing
Enabled from LRED If average retries <
min_th calculate backoff time as usual
If pacing, backoff time increases by a time equal to the transmission time of the previous packet
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Performance
Chain Topology In all cases LRED & Pacing increased TCP
throughput by up to 30%TCP stabilizes at a window size close to the
optimal valueThe longer the chain, the better the
improvement, due to pacing optimizing spatial channel reuse
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Chain Topology
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Performance
Cross Topology Increased throughput and improves fairness (Jain’s) for
both flows TCP NewReno has large unfairness, due to 802.11
capture characteristic (collision of 2 packets, one weaker than the other. The stronger packet is received)
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Performance
Grid TopologyAlso increases throughput and fairness
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Conclusions
Only when buffer is small do buffer overflow drops dominate
As buffer increases, link-layer drops dominate
Link drop acts as a RED gateway, but is insufficient
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Questions