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Low Delay Marking for TCP in Wireless Ad Hoc Networks. Choong-Soo Lee , Mingzhe Li Emmanuel Agu, Mark Claypool, Robert Kinicki Worcester Polytechnic Institute Apr 17, 2004. Introduction. Wireless Ad Hoc Network uses TCP - PowerPoint PPT Presentation
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Low Delay Marking for TCPin Wireless Ad Hoc Networks
Choong-Soo Lee, Mingzhe LiEmmanuel Agu, Mark Claypool, Robert Kinicki
Worcester Polytechnic Institute
Apr 17, 2004
2
Introduction
Wireless Ad Hoc Network uses TCP TCP, being designed for wired networks, performs poorly
over wireless networks. Wireless ad hoc network uses Carrier Sense Multiple
Access with Collision Avoidance (CSMA/CA) and Request-to-Send/Clear-to-Send (RTS/CTS) mechanism to avoid collisions.
TCP performance suffers from the contention delays and drops known as RTS/CTS jamming and RTS/CTS-induced congestion.
3
Introduction
Previous research to improve TCP performance includes Investigation of link breakage and routing failure problems
[4] [5] [6] link layer/MAC solutions [7] [8] [9] protocol modifications [10]
Most of these approaches are link layer optimizations tied to the device drivers rather than the operating system.
4
Motivation
Previous research is only concerned with improved throughput. Emerging applications such as interactive multimedia and
network games demand low round-trip times. End-to-end delays will become increasingly important
relative to throughput. Throughput is important but we project steady increase in
maximum wireless network capacity.
5
Proposal
We propose an IP layer solution which modifies the packet queue manager. Our goal is to improve round-trip times, loss rates and
collisions with minimal degradation to throughput. This facilitates easier deployment since operating system
upgrades/patches can be used independently of hardware changes.
7
Explicit Congestion Notification
Traditionally, TCP uses dropped packets as an indication of network congestion. This requires 3 duplicate acknowledgement. Window size below 4 results in a retransmission timeouts
and reduces throughput significantly.
Explicit Congestion Notification (ECN) uses an unused bit (the ECN bit) in IP header to get congestion notification.
8
Link RED and Adaptive Pacing
Link RED has a similar mechanism to Random Early Detection (RED). It uses an exponentially weighted average of RTS
retries to calculate the dropping/marking probability. Adaptive Pacing is an additional mechanism that
Link RED controls. Adaptive pacing adds extra back-off before trying to
send a packet.
[8]
10
Performance and Window Size
[8] demonstrates that the throughput is not optimal with regular TCP.
Optimal Window Size also provides reasonable delay.
We can adjust the packet marking probability to force TCP to operate around the optimal window size.
11
Low Delay Marking Algorithm
At each node, on receiving packet p identify flow fi to which p belongs
estimate hi for fi estimate n calculate wopt calculate pmark mark p with
probability pmark
p : packetfi : the i-th flowhi : the number of wireless hopsn : the total number of flows going through the nodewopt : the optimal window size for fi
pmark : the marking probability
12
Optimal Window Size
Optimal window size is a function of the number of hops between the source and destination nodes. Due to the hidden terminal problem, it is derived that there
should be only one packet in transit every 4 hops for optimal TCP throughput.
n
h
wopt4
13
Number of Hops
We use Time-To-Live (TTL) values in the data packets. The default TTL values are typically 128 or 256. We keep track of source and destination node pairs to
identify each flow. We take the TTL values from the packets going one way
and the packets going the other way. We subtract them from the default TTL values and sum the
difference.
14
Number of Flows
We estimate the number of flows using Morris’ calculation. We use a fixed-length bit v. A packet is hashed based on source-destination address
and port number and the corresponding bit in v is set. The bits in v are cleared at a certain rate and also the
corresponding number of hops.
[17]
15
Marking Probability
We use Morris’ formula that links the overall loss rate and the TCP window size. We consider the overall loss rate as an equivalent of
marking probability.
Then we substitute all the previous calculated/estimated values to come up with
2
76.0
wp
2
2
2
16.12
4
76.0
h
n
nh
pmark
16
Marking Probability
However, this is the overall marking probability, NOT per-node. We distribute the overall marking probability uniformly along
all nodes.
1
1
2
2
1
16.121
11
h
node
hnodemark
h
np
pp
18
Simulation Setup
We used NS-2 to implement and evaluate LDM. LRED and Adaptive Pacing implementations LDM implementations (hard-coded version)
Wireless Multi-hop Chain Network For h-hop network, we need h+1 nodes (n0 to nh).
All TCP flows go from n0 to nh. We tested 7-hop, 15-hop and 24-hop networks. All TCP flows use TCP NewReno.
19
Single Flow Experiment
Roundtrip Time
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
5 10 15 20 25
Number of Hops
Ro
un
dtr
ip T
ime
(s)
Original TCP Restrained TCP Adaptive Pacing LDM
20
Single Flow Experiment
Normalized Throughput
0.9
1.0
1.1
1.2
1.3
1.4
5 10 15 20 25
Number of Hops
No
rmal
ized
Th
rou
gh
pu
t
Original TCP Restrained TCP Adaptive Pacing LDM
21
Multiple Flow Experiment
Roundtrip Time
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
5 10 15 20 25
Number of Hops
Ro
un
dtr
ip T
ime
(s)
Original TCP Restrained TCP Adaptive Pacing LDM
22
Multiple Flow Experiment
Normalized Throughput
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
5 10 15 20 25
Number of Hops
No
rmal
ized
Th
rou
gh
pu
t
Original TCP Restrained TCP Adaptive Pacing LDM
23
Summary
Category Single Flow Multiple Flows
Hops 7 15 24 7 15 24
Throughput + 0 0 + + 0
Round-Trip Time + + + + + +
Loss Rate + + + + + +
RTS Collisions 0 + + 0 + +
Performance Comparison to Regular TCP
+ : better by more than 10%0 : within 10%– : worse by more than 10%
24
Summary
Category Single Flow Multiple Flows
Hops 7 15 24 7 15 24
Throughput 0 – – 0 – –
Round-Trip Time + + + + + +
Loss Rate 0 0 0 + + +
RTS Collisions + + + + + +
Performance Comparison to Adaptive Pacing
+ : better by more than 10%0 : within 10%– : worse by more than 10%
26
Conclusion
Low Delay Marking (LDM) is an IP layer approach. lowers delay and loss rate without sacrificing throughput.
round-trip time up to 57.6% loss rate up to 59.5%
reduces MAC layer congestion.
We successfully implemented and evaluated Low Delay Marking (LDM) in NS-2.
27
Future Work
All our evaluation is done over with the number of hops and number of flows known ahead of time at each node. Implementation and evaluation of hop and flow counting
techniques
Investigation of LDM performance over more complex topologies such as crosses and grids to evaluate robustness.