TCP Performance and Fairness
over Mobile Ad Hoc Networks
Seok-Hoon Yoon
Index

Introduction
 Issues

of TCP over MANETs
TCP Performance over MANETs
 Cross Layer
 TCP-ELFN

(TCP and network) Approaches
TCP Fairness over MANETs
 Network Layer Approaches
 Neighborhood RED

Conclusion
2/41
Research Trend

TCP performance over MANETs


Most TCP performance studies are based on simulations and
experiments rather than an analytical study
Many approaches

Single layer



Cross layer




TCP
Link, Mac
TCP and network
TCP and physical
Network and physical
TCP fairness over MANETs


Many investigation papers
A few suggestions

Neighborhood RED
3/41
Issues of TCP over MANETs
 Lossy channels
 High bit error rate
 Path asymmetry
 Bandwidth asymmetry
 Loss rate asymmetry



The backward path is much more lossy than the forward path
It may produce bandwidth asymmetry
Route asymmetry


Due to lack of transmission power
Distinct paths for TCP data and TCP ACKs
4/41
Issues of TCP over MANETs
 Network
partition
Due to node mobility and energy constrained
operation
 If disconnectivity > RTO

The TCP sender will trigger exponential backoff
 Doubling the RTO
 After the network is connected again, TCP is still in the
backoff state

5/41
Issues of TCP over MANETs
 Routing



Very frequent events in MANETs
Due to node mobility and repeated transmission failure from
link layer contention
After route re-establishment TCP will face a brutal fluctuation
in RTT
 Power


failures
constraints
Power saving – reducing the power consumption
Power control – adjusting the transmission power of mobile
nodes
6/41
Issues of TCP over MANETs

TCP Congestion Control
 TCP
uses the occurrence of losses to detect
congestion



In MANETs, random wireless errors and mobility serves as
primary contributor to losses as well as congestion
More than 80% of the losses in the network are due to link
failures
Essentially, most losses in ad-hoc networks occur as a result
of route failures
 If
TCP enters congestion control state because of
packet losses caused by random wireless errors and
mobility, then the throughput of TCP can be degraded
significantly
7/41
Why TCP?

Many drawbacks of TCP
 New

Transport Protocol for MANETs?
ATP
Layer Coordination
 Rate Based Transmissions

 TCP
for MANETs?
A large number of application
 Seamless integration with the Internet

8/41
Index

Introduction
 Issues

of TCP over MANETs
TCP Performance over MANETs
 Cross Layer
 TCP-ELFN

(TCP and network) Approaches
TCP Fairness over MANETs
 Network Layer Approaches
 Neighborhood RED

Conclusion
9/41
TCP ELFN (Explicit Link Failure
Notification )




Analysis of TCP performance in static, linear,
multi hop wireless network
Analysis of TCP in MANETs using expected
throughput and measured throughput
Suggestion of TCP ELFN
Simulation results
10/41
TCP performance in simple, static,
linear multi-hop network

A simple multi-hop network

TCP-Reno throughput over an 802.11 fixed, linear,
multi-hop network of varying length
11/41
Performance metric

Performance metric
 Expected
throughput =
Σi=0∞ t i * Ti
Σi=0∞ t i
i: # of hops
ti: the duration for which the shortest path contains i hops
Ti: the throughput obtained “over


a linear chain” using i hops
Expected throughput does not take into account the
performance overhead of determining new routes after
route failures
It serves as a upper bound of throughput in mobile
network
12/41
Performance metric :
Expected throughput

Example
Δt=to
S
Δt=t2
Δt=t1
R
Throughput = TH1
S
R
Throughput = TH3
S
R
Throughput = TH1
Throughput in linear network when # hops is n
t 0*TH1 + t1*TH2 + t2*TH1
Expected throughput =
to + t1 + t2
13/41
Expected throughput and
Measured Throughput

Simulation
environment





ns network simulator
TCP-Reno over 802.11
DSR, BSD’s ARP
30 nodes, 1500X300 m2 ,
the random waypoint
The average throughput
of 50 scenarios
From 2m/s to 10m/s the throughput
drops sharply
14/41
Comparison of measured and expected throughput
for the 50 different Mobility patterns( 2m/s, 10m/s,
20m/s, 30m/s)
15/41
Zero Throughput

T = 0s, route fail, packet dropped
S

R
A
B
C
R
A
B
C
R
T = 18.1xxs, the second retransmission of data packet, dropped
again due to stale cached route
S

C
T = 6.1xxs, ACK dropped, due to stale cached route
S

B
T = 6s, data packet retransmitted
S

A
A
B
C
T=42,90,120s no ACK from the TCP receiver
R
16/41
Some facts





In previous example, only for 6 s of 120 s the network is
partitioned
DSR’s stale cached route can degrade TCP throughput
significantly
DSR does not retransmit dropped packet when it
receives Route Error Msg, and the TCP sender or
receiver does not know about the packet loss
The TCP sender waits for occurring time out
Unnecessary RTO back-off of the TCP sender makes
problems even worse
17/41
TCP ELFN

Explicit Link Failure Notification (ELFN)
 The objective :
 To provide the TCP sender with information about
link and route failures
 TCP sender can avoid responding to the failures as if
congestion occurred
 DSR’s route failure message is modified
 A payload similar to the “host unreachable” ICMP
message
 The sender and receiver’s addresses and ports and
seq number
TCP data
S
A
Probing
message
B
C
DSR ROUTE ERROR
+ ELFN
D
R
18/41
TCP ELFN

Sender reaction
 When

It disables its retransmission timers and enters a
“stanby” mode
 While

 If
a TCP sender receives an ELFN,
on standby,
A packet is sent at periodic intervals to probe the
network to see if a route has been established
an acknowledgment is received,

Then it leaves stanby mode
19/41
Simulation for the 50 different Mobility
patterns( 2m/s, 10m/s, 20m/s, 30m/s)
20/41
Simulation for the different probing intervals
and different window and RTO modification

Different probing interval


If the interval is too large, it delays the discovery of new routes
If the interval is too small, the rapid injection of probes into the
network will cause congestion and lower throughput
21/41
Index

Introduction
 Issues

of TCP over MANETs
TCP Performance over MANETs
 Cross Layer
 TCP-ELFN

(TCP and network) Approaches
TCP Fairness over MANETs
 Network Layer Approaches
 Neighborhood RED

Conclusion
22/41
Unfairness of TCP in MANETs

Significant TCP unfairness in ad hoc wireless
networks
 Channel capture
 Hidden terminal conditions
 The binary exponential backoff

of IEEE 802.11
The RED scheme for wired networks
 Keeps
the queue size relatively small and drops or
marks packets proportional to the bandwidth share
 avg = (1-wq)*avg + wq*q

 It
q: current queue size, wq: queue weight
does not work in wireless ad hoc networks
23/41
RED in MANETs

Simple simulation


3 FTP connections
FTP2 is always starved
24/41
RED in MANETs

Why does not RED work well in MANETs?

A TCP connection penalized in channel contention drop
more packets


Congestion does not happen in a single node


It may actually increase the unfairness
Instead happens in an entire area involving multiple nodes
Multiple nodes should coordinate their “packet drops”,
rather than drop independently
25/41
Neighborhood RED

Overview of NRED





NRED extends the original RED scheme
Each node keeps estimating the size of its neighborhood queue
(distributed queue)
Once the queue size exceeds a certain threshold, an overall
drop probability is computed by the algorithm of RED
This overall drop probability is then propagated to neighboring
nodes for cooperative packet drops
However, there is no real distributed queue in ad hoc
network, so how to implement distributed queue?
26/41
Neighborhood and Its Distributed
Queue

Neighborhood
 A node’s
neighborhood consists of the node itself and
the nodes which can interfere with this node’s signals

Distributed Queue of a
Node



The outgoing queue of the
node itself
1-hop neighbors' outgoing
queues
2-hop neighbors’ packets
which are directed to a 1-hop
neighbor of node A
A node’s Neighborhood and it’s
distributed queue 27/41
A Simplified Neighborhood Queue
Model

Simplified Model

2-hop neighborhood distributed
queue model is not easy to
implement and evaluate


A lot of control packet
overhead
The packets in the 2-hop
neighbors directed to a 1-hop
neighbor are moved to the 1hop neighbor


Outgoing queue – the original
queue at a node
Incoming queue – the packets
from 2-hop neighbors
28/41
Neighborhood Random Early
Detection (NRED)

3 problems to solve
 How to detect the early congestion of a neighborhood?
 Neighborhood Congestion Detection (NCD)
 When
and how does a node inform its neighbors
about the congestions?

Neighborhood Congestion Notification (NCN)
 How
do the neighbor nodes calculate their local drop
probabilities?

Distributed Neighborhood Packet Drop (DNPD)
29/41
Neighborhood Congestion
Detection (NCD)

A direct way to monitor the neighborhood queue size



Every node broadcast a control packet to announce its queue
size
A lot of control overhead will be caused
A passive measurement technique

An alternate measure related to queue size


A relationship between channel utilization and the size of both
outgoing and incoming queues


Channel utilization
When these queues are busy, channel utilization around the node is
more likely to increase
How to know the channel utilization of neighborhood?
30/41
Neighborhood Congestion
Detection (NCD)

A passive measurement technique
data
A
A packet in outgoing queue is
transmitted
CTS
A
A packet is received to any
incoming queue
31/41
Neighborhood Congestion
Detection (NCD)

A node monitors five different radio state






Transmitting (Ttx)
Receiving (Trx)
Carrier sensing busy (Tcs)
Virtual carrier sending busy (Tvcs)
Idle (Tidle)
By monitoring the five radio states, a node can now estimate 3
channel utilization ratio
Total channel utilization Ubusy = Tinterval - Tidle
Tinterval
Ttx
 Transmitting ratio Utx =
Tinterval
Trx
 Receiving ratio Urx =
Tinterval


Tinterval = Ttx + Trx + Tcs + Tvcs + Tidle

Ubusy reflects the size of the neighborhood queue
Utx and Urx reflect the channel bandwidth usage of the outgoing queue and incoming queue at current
node
32/41

Neighborhood Congestiond
Detection (NCD)

To facilitate the implementation of the RED algorithm, the
channel utilization is translated into an index of the
queue size
Ubusy * W

The queue size index q =
C
W:channel bandwidth, C: the average packet size

Now the original RED scheme can be applied

The average queue size ,


avg = (1-wq)*avg + wq*q
If the queue size exceeds a certain threshold, the
neighborhood is in congestion
33/41
Neighborhood Congestiond
Notification (NCN)

Drop probability
 Pb
=
Maxp* (Avg – Minth)
Maxth - Minth
 Normalized

Current node A broadcasts Drop probability to 1hop neighbors
 The

Pb = Pb/avg
broadcast message  drop probability + life time
Neighborhood nodes choose the largest drop
probability, if they receive multiple NCN
34/41
Distributed Neighborhood Packet
Drop

Each node calculate its “share” of this overall
drop probability according to its channel
bandwidth usage
 Pb_local =
 Incoming

queue drop probability
Pb_lncoming =
 Outgoing

Pb*(avgtx + avgrx)
avg
Pb * avgrx
avg
queue drop probability
Pb_Outgoing =
Pb * avgtx
avg
35/41
Verification of queue size
estimation
Estimated Queue Size
Real Queue Size
<<Scenario>>
<<Queue size of Node 5>>
36/41
Simulations – Previous Scenario
maxp = 0.14
37/41
Simulations – Multiple congested
neighborhood
1. Dropped packets already used the channel bandwidth
2. NRED tends to keep the wireless channel underutilized
38/41
Simulations – Mobility
<<Scenario>>
39/41
Conclusion



The standard TCP is optimized in context of wired
networks
Several issues of TCP over MANETs and characteristics
of TCP in MANETs has been introduced
In MANETs, the standard TCP shows poor performance



In MANETs, packet losses is usually caused by high bit error
rate, route failures as well as congestion
To avoid to enter the TCP congestion control on route change,
several improvements have been proposed
The very poor fairness is shown by the standard TCP in
MANETs

For better TCP fairness, NRED has been proposed
40/41
References






K. Xu, M. Gerla, L. Qi, and Y. Shu, “Enhancing TCP fairness in ad hoc
wireless networks using neighborhood red,” in Proc. of ACM MOBICOM,
San Diego, CA, USA, Sep. 2003, pp. 16–28.
K. Sundaresan, V. Anantharaman, H.-Y. Hsieh, and R. Sivakumar. ATP:
A reliable transport protocol for ad-hoc networks. In Proceedings of 4th
ACM MobiHoc, pp. 64–75, 2003.
G. Holland and N. Vaidya, “Analysis of TCP performance over mobile ad
hoc networks,”ACM Wireless Networks, vol. 8, no. 2, pp. 275–288, Mar.
2002.
Z. Fu, X. Meng, and S. Lu. How bad TCP can perform in mobile ad hoc
networks. In Proceedings of 7th IEEE ISCC, 2002.
V. Anantharaman and R. Sivakumar. A microscopic analysis of TCP
performance over wireless ad-hoc networks.Presented in 2nd ACM
SIGMETRICS (Poster Paper), 2002.
F. Wang and Y. Zhang. Improving TCP performance over mobile ad-hoc
networks with out-of-order detection and response. In Proceedings of
3rd ACM MobiHoc, pp. 217–225, 2002.
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