TCP in Mobile Ad-hoc Networks - Split TCP

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TCP in Mobile Ad-hoc Networks - Split TCP

• CSE 6590

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Overview

• What is TCP?
• TCP challenges in MANETs
• TCP-based solutions
• Split-TCP
• ATCP

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TCP A Brief Review

• TCP Transmission Control Protocol
• Specified in 1974 (TCP Tahoe)
• Data stream ? TCP packets
• Reliable end-to-end connection
• In-order packet delivery
• Flow and congestion control

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How does TCP work?

• Establishes an end-to-end connection
• Acknowledgement based packet delivery
• Assigns a congestion window Cw
• Initial value of Cw 1 (packet)
• If trx successful, congestion window doubled.
  Continues until Cmax is reached
• After Cw Cmax, Cw Cw 1
• If timeout before ACK, TCP assumes congestion

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How does TCP work? (2)

• TCP response to congestion is drastic
• A random backoff timer disables all transmissions
  for duration of timer
• Cw is set to 1
• Cmax is set to Cmax / 2
• Congestion window can become quite small for
  successive packet losses.
• Throughput falls dramatically as a result.

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TCP Congestion Window
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Why does TCP perform badly in MANETs?

• Dynamic network topology
• Node mobility
• Network partition
• Multi-hop paths
• Variable path lengths
• Longer path higher failure rate

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Why does TCP struggle in MANETs? (2)

• Lost packets due to high BER (Bit Error Rate)
• BER in wired 10-8 10-10
• BER in wireless 10-3 10-5

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Solutions for TCP in MANETs

• Various solutions present
• Most solutions generally tackle a subset of the
  problem
• Often, fixing one part of TCP breaks another part
• Competing interests exist in the standards laid
  out by OSI

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Solution Topology
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Why focus on TCP-based solutions?

• We want to choose solutions which maintain close
  connection to TCP
• Upper layers in the OSI model affected by choice
  of transport layer protocol
• Modifications may affect interactions with the
  Internet
• Alternative methods only useful for isolated
  networks

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Solutions for TCP
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Split-TCP and ATCP
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TCP Summary

• Works well in wired
• Fails in wireless networks due to frequent
  connection breaks
• Mobile nodes move
• Packets lost due to lossy channels
• Multi-hop paths more prone to failure
• Present solutions tackle subset of problems
• Two solutions Split-TCP and ATCP

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Split-TCP Overview

• Motivation for Split-TCP
• How does Split-TCP work?
• Advantages/Disadvantages
• Performance Evaluation
• Throughput vs. TCP
• Channel Capture Effect
• Summary

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Split-TCP in Solution Topology
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Motivation for Split-TCP

• Issues addressed by Split-TCP
• Throughput degradation with increasing path
  length
• Channel capture effect (802.11)
• Mobility issues with regular TCP

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Channel Capture Effect

• Definition
• The most data-intense connection dominates the
  multiple-access wireless channel 1
• Higher SNR
• Early start
• Example 2 simultaneous heavy-load TCP flows
  located close to each other.

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How does Split-TCP work?

• Connection between sender and receiver broken
  into segments
• A proxy controls each segment
• Regular TCP is used within segments
• Global end-to-end connection with periodic ACKs
  (for multiple packets)

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Split-TCP Segmentation
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Split-TCP in a MANET Proxy Functionality

• Proxies
• Intercept and buffer TCP packets
• Transmit packet, wait for LACK
• Send local ACK (LACK) to previous proxy
• Packets cleared upon reception of LACK
• Increase fairness by maintaining equal connection
  length

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Split-TCP in a MANET (2)

• Steps
• Node 1 initiates TCP session
• Nodes 4 and 13 are chosen as proxies on-demand
• Upon trx, 4 buffers packets
• If a packet lost at 15, request made to 13 to
  retransmit
• 1 unaware of link failure at 15

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Split-TCP in a MANET (3)

• Sender is unaware of transient link failure.
  Congestion window not reduced.
• Packet retransmissions only incorporate part of a
  path ? bandwidth usage is reduced.
• Channel capture effect is alleviated (see next
  slide).

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Channel capture alleviated
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Is Split-TCP successful?

• Pros
• Increased throughput
• Increased fairness
• Restricted channel capture effect
• Cons
• Modified end-to-end connection
• Proxy movement/failure adversely affects protocol
  performance
• Congestion at proxy nodes if another fails

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Performance Evaluation

• Test bench Specifics
• ns-2 Simulator
• 50 mobile nodes initially equidistant
• 1 km2 Area
• Nodes maintain constant velocity
• Arbitrary direction
• Random changes at periodic intervals
• Optimal segment length 3 n 5 nodes
• Measured improvement Throughput increases by 5
  to 30

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Performance vs. TCPThroughput Comparison
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Performance vs. TCPChannel Capture Effect
Regular TCP Throughput
Split-TCP Throughput
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Split-TCP Summary

• Break link into segments with proxies
• Use proxies to buffer packets at segments
• Employ TCP locally in segments
• Reduce bandwidth consumption and channel capture
  effect

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Issues Not Addressed

• Does not maintain end-to-end semantics
• Periodic ACK failures means major retransmissions
• Packet loss due to high BER
• Out-of-order packets
• Proxy link failure affects performance

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References

• 1 Split-TCP for Mobile Ad Hoc Networks
  Kopparty et al.
• 2 ATCP TCP for Mobile Ad Hoc Networks Jian
  Liu, Suresh Singh, IEEE Journal, 2001.
• 3 A Feedback-Based Scheme for Improving TCP
  Performance in Ad Hoc Wireless Networks Kartik
  Chandran et al.
• 4 Ad Hoc Wireless Networks Architectures and
  Protocols C. Siva Ram Murthy and B. S. Manoj
  section 9.5.7.
• 5 Improving TCP Performance over Wireless
  Networks Kenan Xu, Queens University 2003.