TCP Performance and Fairness over Mobile Ad Hoc Networks

1
TCP Performance and Fairness over Mobile Ad Hoc
Networks

• Seok-Hoon Yoon

2
Index

• Introduction
• Issues of TCP over MANETs
• TCP Performance over MANETs
• Cross Layer (TCP and network) Approaches
• TCP-ELFN
• TCP Fairness over MANETs
• Network Layer Approaches
• Neighborhood RED
• Conclusion

3
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
• TCP
• Link, Mac
• Cross layer
• TCP and network
• TCP and physical
• Network and physical
• TCP fairness over MANETs
• Many investigation papers
• A few suggestions
• Neighborhood RED

4
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

5
Issues of TCP over MANETs

• Network partition
• Due to node mobility and energy constrained
  operation
• If disconnectivity gt RTO
• The TCP sender will trigger exponential backoff
• Doubling the RTO
• After the network is connected again, TCP is
  still in the backoff state

6
Issues of TCP over MANETs

• Routing failures
• 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 constraints
• Power saving reducing the power consumption
• Power control adjusting the transmission power
  of mobile nodes

7
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

8
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

9
Index

• Introduction
• Issues of TCP over MANETs
• TCP Performance over MANETs
• Cross Layer (TCP and network) Approaches
• TCP-ELFN
• TCP Fairness over MANETs
• Network Layer Approaches
• Neighborhood RED
• Conclusion

10
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

11
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

12
Performance metric

• Performance metric
• Expected throughput

t i Ti
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

13
Performance metric Expected throughput

• Example

?tt2
?tto
?tt1
S
R
S
R
S
R
Throughput TH1
Throughput TH3
Throughput TH1
Throughput in linear network when hops is n
Expected throughput
t 0TH1 t1TH2 t2TH1
to t1 t2
14
Expected throughput and Measured Throughput

• Simulation environment
• ns network simulator
• TCP-Reno over 802.11
• DSR, BSDs ARP
• 30 nodes, 1500X300 m2 , the random waypoint
• The average throughput of 50 scenarios

From 2m/s to 10m/s the throughput drops sharply
15
Comparison of measured and expected throughput
for the 50 different Mobility patterns( 2m/s,
10m/s, 20m/s, 30m/s)
16
Zero Throughput

• T 0s, route fail, packet dropped

S
A
B
C
R

• T 6s, data packet retransmitted

S
A
B
C
R

• T 6.1xxs, ACK dropped, due to stale cached route

S
A
B
C
R

• T 18.1xxs, the second retransmission of data
  packet, dropped again due to stale cached route

S
A
B
C
R

• T42,90,120s no ACK from the TCP receiver

17
Some facts

• In previous example, only for 6 s of 120 s the
  network is partitioned
• DSRs 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

18
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
• DSRs route failure message is modified
• A payload similar to the host unreachable ICMP
  message
• The sender and receivers addresses and ports and
  seq number

TCP data
R
S
A
B
C
D
DSR ROUTE ERROR ELFN
Probing message
19
TCP ELFN

• Sender reaction
• When a TCP sender receives an ELFN,
• It disables its retransmission timers and enters
  a stanby mode
• While on standby,
• A packet is sent at periodic intervals to probe
  the network to see if a route has been
  established
• If an acknowledgment is received,
• Then it leaves stanby mode

20
Simulation for the 50 different Mobility
patterns( 2m/s, 10m/s, 20m/s, 30m/s)
21
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

22
Index

• Introduction
• Issues of TCP over MANETs
• TCP Performance over MANETs
• Cross Layer (TCP and network) Approaches
• TCP-ELFN
• TCP Fairness over MANETs
• Network Layer Approaches
• Neighborhood RED
• Conclusion

23
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 wqq
• q current queue size, wq queue weight
• It does not work in wireless ad hoc networks

24
RED in MANETs

• Simple simulation
• 3 FTP connections
• FTP2 is always starved

25
RED in MANETs

• Why does not RED work well in MANETs?
• A TCP connection penalized in channel contention
  drop more packets
• It may actually increase the unfairness
• Congestion does not happen in a single node
• Instead happens in an entire area involving
  multiple nodes
• Multiple nodes should coordinate their packet
  drops, rather than drop independently

26
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?

27
Neighborhood and Its Distributed Queue

• Neighborhood
• A nodes neighborhood consists of the node itself
  and the nodes which can interfere with this
  nodes 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 nodes Neighborhood and its distributed queue
28
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 1-hop neighbor
• Outgoing queue the original queue at a node
• Incoming queue the packets from 2-hop
  neighbors

29
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)

30
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
• Channel utilization
• A relationship between channel utilization and
  the size of both outgoing and incoming queues
• When these queues are busy, channel utilization
  around the node is more likely to increase
• How to know the channel utilization of
  neighborhood?

31
Neighborhood Congestion Detection (NCD)

• A passive measurement technique

data
CTS
A
A
A packet is received to any incoming queue
A packet in outgoing queue is transmitted
32
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
• Transmitting ratio Utx
• Receiving ratio Urx
• 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

33
Neighborhood Congestiond Detection (NCD)

• To facilitate the implementation of the RED
  algorithm, the channel utilization is translated
  into an index of the queue size
• The queue size index q
• 
  Wchannel bandwidth, C the average packet size
• Now the original RED scheme can be applied
• The average queue size ,
• avg (1-wq)avg wqq
• If the queue size exceeds a certain threshold,
  the neighborhood is in congestion

34
Neighborhood Congestiond Notification (NCN)

• Drop probability
• Pb
• Normalized Pb Pb/avg
• Current node A broadcasts Drop probability to
  1-hop neighbors
• The broadcast message ? drop probability life
  time
• Neighborhood nodes choose the largest drop
  probability, if they receive multiple NCN

35
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 queue drop probability
• Pb_Outgoing

36
Verification of queue size estimation
Estimated Queue Size
Real Queue Size
ltltQueue size of Node 5gtgt
ltltScenariogtgt
37
Simulations Previous Scenario
maxp 0.14
38
Simulations Multiple congested neighborhood

1. Dropped packets already used the channel
   bandwidth
2. NRED tends to keep the wireless channel
   underutilized

39
Simulations Mobility
ltltScenariogtgt
40
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

41
References

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