An End-to-end Approach to Increase TCP Throughput Over Ad-hoc Networks Sarah Sharafkandi and Naceur...
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Transcript of An End-to-end Approach to Increase TCP Throughput Over Ad-hoc Networks Sarah Sharafkandi and Naceur...
An End-to-end Approach to Increase TCP Throughput Over Ad-hoc Networks
Sarah Sharafkandi and Naceur Malouch
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Introduction
TCP is designed for wired networks Congestion control : window-based
With IEEE 802.11 PHY & MAC, TCP over Ad-hoc has a low performance: congestion control and not “collision” control:
TCP react to buffer overflow "bursty" traffic inherent reverse traffic
Objective: Improve TCP throughput without modifying PHY, MAC and NET layers.
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When collision causes DATA loss?
By hidden nodes: packets sent by D collide with A’s packets at node B preventing B from decoding A’s packets.
By repetitive retries due to “ordinary” collisions: it happens when C* rare event
By buffer overflow : due to increased waiting times not considered in this work
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State of the art Distributed Link RED and Adaptive pacing [Fu et al.
INFOCOM’2003] If the average number of retransmission retry > min_thresh :
early drop of packets increase the backoff period
Improvement: 10%-30% for the chain topology Increasing retry limit and optimum packet size [Jiang et al.
DISCEX’O3] Increasing the retry limit reduces oscillations in the instantaneous thpt Increasing the packet size increases the thpt till some thresh
Improving TCP throughput using Delayed ack method [Altman et al. MADNET’03] delayed ack factor = 2, 3
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Simulation Scenario
NS2 network simulator Chain topology The source and destination at both ends of the
chain AODV as a routing protocol Some modifications to the source code of NS2:
delayed ack > 2 monitoring without file traces token bucket: packet version
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TCP Sends the packets in “burst”
Two experiments to show the effect of “burstiness” Simulation with TCP using RFC3465 Simulation with CBR traffic
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Simulation with TCP using RFC3465
The “burstiness” of RFC3465 results in throughput reduction despite the gain in the window growth
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Simulation with CBR traffic: Results
Best result is when there is packet spacing “burstiness” is minimum
i CBR traffics with rate r/i, i = 1, 2, 3, 4.
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New approach
Bursty data traffic over Ad-hoc networks results to performance reduction
Shaping : Controls the rate of releasing packets to the network No more aggressive traffic Plus delayed ack approaches the optimal channel
reuse
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Optimum rate
There is always an optimum rate for the shaper in which TCP has the best performance
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TCP throughput as a function of Number of hops
Optimum rate decreases when number of hops increases
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Impact of bucket size
A data can pass through the shaper only if it can get a token from token buffer.
We can use it to test again the effect of burstiness
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Effectiveness of Shaping in presence of CBR Traffic
Network scenario : same source/destination for UDP traffic
UDP share all the ad-hoc routers with TCP Compute the gain while increasing the rate of UDP:
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Conclusion
TCP throughput drops significantly because of: link contention caused by hidden terminal problem An "aggressive“ TCP sender causes an increased contention at
the MAC layer
Implementing a shaper at the sender improves TCP throughput by controlling the aggression of TCP data traffic
Delayed ack mechanism plus the shaper
→ increase spatial channel reuse
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Future work An adaptive algorithm for finding the optimum rate
difficulties: convergence and stability Related work: [ElRakabawy et al. MobiHoc’2005]
same idea: end-to-end solution BUT :
change TCP protocol for the multihop wireless ad-hoc based on the esimation of the 4-hop transmission delay
Our approach :