1 transport layer for mobile ad hoc networks (manets) cyrus minwalla maan musleh cosc 6590
TRANSCRIPT
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Transport Layer for Mobile Ad Hoc Networks
(MANETs)
Cyrus MinwallaMaan MuslehCOSC 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?
1. Dynamic network topology Nodes in constant motion Network Topology undergoes
periodic changes
2. Multi-hop paths Variable path lengths per node Longer path = higher failure rate
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Why does TCP struggle in MANETs? (2)
3. 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 --> 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. TCP:Throughput Comparison
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Performance vs. TCP:Channel 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
IP
TCP
ATCP
IP
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How ATCP works (1) - lossy channel
Disconnected *
Congested
Normal
Loss *
* TCP sender in persist state
RTO aboutTo expire OR3 dup ACKs
NewACK
ATCP RetransmitsSegments in buffer
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How ATCP works (2) - Congestion
Disconnected *
Congested
Normal
Loss *
ReceiveECN TCP Transmits
a new packet
* TCP sender in persist state
RTO aboutTo expire OR3 dup ACKs
NewACK
ATCP RetransmitsSegments in buffer
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How ATCP works (3) - Node mobility
Disconnected *
Congested
Normal
Loss *
Receive “Dest Unreachabl” ICMP
ReceiveECN TCP Transmits
a new packet
Receive Dup ACK or packet from receiver
* TCP sender in persist state
RTO aboutTo expire OR3 dup ACKs
NewACK
ATCP RetransmitsSegments 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, Queen’s University 2003
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The End
Thank you for your patience
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Questions/Comments?