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Transport Layer for Mobile Ad Hoc Networks (MANETs)

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Modifications may affect interactions with the Internet ... How does Split-TCP work? Connection between sender and receiver broken into segments ... – PowerPoint PPT presentation

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Title: Transport Layer for Mobile Ad Hoc Networks (MANETs)


1
Transport Layer for Mobile Ad Hoc Networks
(MANETs)
  • Cyrus Minwalla
  • Maan Musleh
  • COSC 6590

2
Overview
  • What is TCP?
  • TCP Challenges in MANETs
  • TCP Based Solutions
  • Split-TCP
  • ATCP
  • Recap

3
What is TCP?
  • Sub-topics
  • Transport Layer overview
  • TCP Summary
  • Solutions
  • Recap

4
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

5
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

6
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

7
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.

8
TCP Congestion Window
9
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

10
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

11
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

12
Solution Topology
13
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

14
Solutions for TCP
15
Split-TCP and ATCP
16
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

17
Split-TCP Overview
  • Motivation for Split-TCP
  • How does Split-TCP work?
  • Advantages/Disadvantages
  • Performance Evaluation
  • Throughput vs. TCP
  • Channel Capture Effect
  • Recap

18
Split-TCP in Solution Topology
19
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

20
Channel Capture Effect
  • Definition
  • The most data-intense connection dominates the
    multiple-access wireless channel 1
  • Higher SNR
  • Early Start

21
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)

22
Split-TCP Segmentation
23
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

24
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

25
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.

26
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)

27
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

28
Performance vs. TCPThroughput Comparison
29
Performance vs. TCPChannel Capture Effect
Regular TCP Throughput
Split-TCP Throughput
30
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

31
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

32
ATCP Overview
  • What is ATCP?
  • Motivation for ATCP
  • ATCP Infrastructure
  • How ATCP works
  • Is ATCP Successful?
  • Performance vs. TCP
  • ATCP Recap

33
What is ATCP?
  • Overview
  • Ad Hoc TCP
  • Network Layer Feedback Mechanism
  • TCP State Control
  • End-to-end Semantics
  • Dependent on routing protocols

34
ATCP in Solution Topology
35
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

36
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
37
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
38
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
39
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
40
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

41
Performance vs. TCP (File Transfer Time)
42
Performance vs. TCP (2)(Congestion Window Size)
43
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

44
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

45
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

46
The End
  • Thank you for your patience

47
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