Transport Layer for Mobile Ad Hoc Networks (MANETs)

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Transport Layer for Mobile Ad Hoc Networks
(MANETs)

• Cyrus Minwalla
• Maan Musleh
• COSC 6590

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Overview

• What is TCP?
• TCP Challenges in MANETs
• TCP Based Solutions
• Split-TCP
• ATCP
• Recap

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What is TCP?

• Sub-topics
• Transport Layer overview
• TCP Summary
• Solutions
• Recap

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Transport Layer

• In the OSI model, the transport layer is
  responsible for
• Reliable end-to-end connection
• End-to-end delivery
• Flow control
• Congestion control
• In-order packet delivery

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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 tx 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 struggle in MANETs?

• Dynamic network topology
• Nodes in constant motion
• Network Topology undergoes periodic changes
• Multi-hop paths
• Variable path lengths per node
• 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 Recap

• Works well in wired
• Fails in wireless due to frequent connection
  breaks
• Mobile nodes being rerouted
• Packets lost due to lossy channel
• 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
• Recap

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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

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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 rx, 4 buffers packet
• If 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
  link --gt Bandwidth reduced
• 4 may act as proxy for 12 as well, channel
  capture eliminated.

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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 adversely affects protocol
  performance
• Congestion at individual nodes (if only proxy
  between partitions)

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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 Recap

• 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 failure means major retransmission
• Packet loss due to high BER
• Out-of-order packets
• Proxy link failure affects performance

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ATCP Overview

• What is ATCP?
• Motivation for ATCP
• ATCP Infrastructure
• How ATCP works
• Is ATCP Successful?
• Performance vs. TCP
• ATCP Recap

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What is ATCP?

• Overview
• Ad Hoc TCP
• Network Layer Feedback Mechanism
• TCP State Control
• End-to-end Semantics
• Dependent on routing protocols

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ATCP in Solution Topology
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Motivation for ATCP

• Issues addressed by ATCP
• Packet loss due to high BER or collision
• Route changes
• Network partitions
• Out-of-Order Packets
• Congestion
• CWND

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ATCP infrastructure

• ATCP is a thin layer that is layered between TCP
  and IP
• Sender ATCP states
• Normal, Disconnected, Congested, and Loss

TCP
TCP
ATCP
IP
IP
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How ATCP works (1) - lossy channel
Disconnected

Normal
New ACK
RTO about To expire OR 3 dup ACKs
Loss
Congested
TCP sender in persist state
ATCP Retransmits Segments in buffer
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How ATCP works (2) - Congestion
Disconnected

Normal
Receive ECN
TCP Transmits a new packet
New ACK
RTO about To expire OR 3 dup ACKs
Loss
Congested
TCP sender in persist state
ATCP Retransmits Segments in buffer
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How ATCP works (3) - Node mobility
Disconnected
Receive Dup ACK or packet from receiver
Receive Dest Unreachabl ICMP

Normal
Receive ECN
TCP Transmits a new packet
New ACK
RTO about To expire OR 3 dup ACKs
Loss
Congested
TCP sender in persist state
ATCP Retransmits Segments in buffer
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Is ATCP Successful?

• Pros
• Maintenance of end-to-end TCP semantics
• Compatibility with traditional TCP
• Invisibility to TCP
• Cons
• Dependency on the network layer protocol to
  detect route changes and partitions
• Addition of a thin ATCP layer to TCP

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Performance vs. TCP (File Transfer Time)
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Performance vs. TCP (2)(Congestion Window Size)
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ATCP Recap

• Introduces a thin layer between IP and TCP
• Maintain End-to-End Semantics
• Does not interfere with TCP functions
• Depends on the Network Layer to detect route
  changes and partitions

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Final Recap

• TCP does not perform well in MANETs
• The presented solutions fix various aspects of
  TCP.
• Currently there is no comprehensive solution that
  fixes all the problems
• Applications are requirement specific

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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
• 5 Improving TCP Performance over Wireless
  Networks Kenan Xu, Queens University 2003

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The End

• Thank you for your patience

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Questions/Comments?