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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?
1. Dynamic network topology Node mobility Network partition
2. Multi-hop paths Variable path lengths 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 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. 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: 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, Queen’s University 2003.