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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?