Download Split-TCP for Mobile Ad Hoc Networks

Transport Layer for Mobile
Ad Hoc Networks (MANETs)
Cyrus Minwalla
Maan Musleh
COSC 6590
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Overview
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What is TCP?
TCP Challenges in MANETs
TCP Based Solutions
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Split-TCP
ATCP
Recap
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What is TCP?
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Sub-topics:
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Transport Layer overview
TCP Summary
Solutions
Recap
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Transport Layer
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In the OSI model, the transport layer is
responsible for:
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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
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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?
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Establishes an end-to-end connection:
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Acknowledgement based packet delivery
Assigns a congestion window Cw:
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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)
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TCP response to congestion is drastic:
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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
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Nodes in constant motion
Network Topology undergoes periodic
changes
2. Multi-hop paths
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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):
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BER in wired: 10-8 – 10-10
BER in wireless: 10-3 – 10-5
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Solutions for TCP in MANETs
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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?
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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
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Works well in wired
Fails in wireless due to frequent connection
breaks:
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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:
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Motivation for Split-TCP
How does Split-TCP work?
Advantages/Disadvantages
Performance Evaluation:
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Throughput vs. TCP
Channel Capture Effect
Recap
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Split-TCP in Solution Topology
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Motivation for Split-TCP
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Issues addressed by Split-TCP:
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Throughput degradation with increasing
path length
Channel Capture effect (802.11)
Mobility issues with regular TCP
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Channel Capture Effect
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Definition:
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“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?
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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
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Proxies:
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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)
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Steps:
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Node 1 initiates TCP
session
Nodes 4 and 13 are
chosen as proxies ondemand
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)
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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?
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Pros:
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Increased throughput
Increased fairness
Restricted channel capture effect
Cons:
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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
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Test bench Specifics:
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ns-2 Simulator
50 mobile nodes initially equidistant
1 km2 Area
Nodes maintain constant velocity:
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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
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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
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Does not maintain end-to-end
semantics
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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:
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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?
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Overview:
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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
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Issues addressed by ATCP:
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Packet loss due to high BER or collision
Route changes
Network partitions
Out-of-Order Packets
Congestion
CWND
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ATCP infrastructure
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ATCP is a thin layer that is layered
between TCP and IP
TCP
IP
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TCP
ATCP
IP
Sender ATCP states:
Normal, Disconnected, Congested, and Loss
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How ATCP works (1)
- lossy channel
Disconnected
*
Normal
Congested
* TCP sender in persist state
RTO about
To expire OR
3 dup ACKs
New
ACK
Loss *
ATCP Retransmits
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Segments in buffer
How ATCP works (2)
- Congestion
Disconnected
Receive
ECN
TCP Transmits
a new packet
Congested
* TCP sender in persist state
*
Normal
RTO about
To expire OR
3 dup ACKs
New
ACK
Loss *
ATCP Retransmits
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Segments in buffer
How ATCP works (3)
Receive “Dest
Unreachabl” ICMP
Receive
ECN
TCP Transmits
a new packet
Congested
* TCP sender in persist state
- Node mobility
Disconnected
*
Normal
RTO about
To expire OR
3 dup ACKs
Receive Dup ACK
or packet from receiver
New
ACK
Loss *
ATCP Retransmits
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Segments in buffer
Is ATCP Successful?
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Pros:
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Maintenance of end-to-end TCP semantics
Compatibility with traditional TCP
Invisibility to TCP
Cons:
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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
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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
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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
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[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?
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