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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An end-to-end approach to “collision control” ?!

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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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Throughput of TCP with shaper and delayed ack

Shaper increases the TCP throughput by 53%-120%

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Shaper and Delayed ack

Shaper allow delayed ack mechanism to bypass the limit of d=3

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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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Tokens

Again allowing “burstiness” results to throughput reduction

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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 :