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1 Transport Layer for Mobile Transport Layer for Mobile Ad Hoc Networks Ad Hoc Networks Prepared By : Prepared By : Patel Jay C Patel Jay C ME(EC)-140870705004 ME(EC)-140870705004

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Transport Layer for Mobile Transport Layer for Mobile Ad Hoc NetworksAd Hoc Networks

Prepared By :Prepared By :

Patel Jay CPatel Jay C

ME(EC)-140870705004ME(EC)-140870705004

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The Quest for the Transport Layer Assess the state of the art of transport protocols

Target environment: mobile ad hoc nets (MANET) Which is the best TCP variant? Do we need a new transport protocol? The question is very timely…

Introduction

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Network Architecture at a Crossroads The community recognizes the need for change Wireline-centric network design is “obsolete” New network environments have emerged

Ad hoc, sensors, consumer-owned, delay-tolerant

New networking technologies have emerged UWB, cooperative approaches, MIMO, directed

antennas

Introduction

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New Class of Networks

Thousands of nodes, highly resource constrained, highly unreliable wireless links, low duty cycle (smartdust)

Tens - thousands of nodes, Nano-sensors

Hundreds of nodes, resource constrained, unreliable wireless links (Sensors)

Tens of nodes, resource constrained, wireless links, charged every day (PDAs)

Tens of nodes, resource constrained, wireless

links, line powered (embedded devices)

Tens of nodes, resource constrained, wireless

links, line powered (computers)

Introduction

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A New Era Has BegunNew Machines

New Environments Applications

New Networks

Introduction

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The Role of Networking is Central

WirelessNetworking

EmbeddedSystems

Sensors

Embedded Sensor

Applications

Introduction

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Revisiting the Architecture

The vision:Wireless as an integral part of the networkMultiple wireless hops: not just the last milePockets of wireless ad hoc connectivity

A new protocol stack is required Is TCP/IP capable of delivering?

Introduction

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Revisiting The Hourglass

Introduction

Email WWW Voice...

SMTP HTTP RTP...

TCP UDP

IP

Ethernet PPP…

MultiAccess async sync...

copper fiber radio...

User Application

Application Protocol

Transport Protocol

Media Access Protocol

Media Sharing Principles

Physical System

Internet Protocol

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Problem: Evaluate TCP

Why does TCP perform poorly in MANETs?Developed for wire-line networks.Assume all losses are due to congestion.

Many TCP variants have been proposed.How good are they? Are they sufficient?

Are there any other alternatives?Are non-TCP protocols the solution?

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Our goal

Identify the problems of TCP in MANETs. Evaluate various major TCP variants.

12 TCP variants, 7 improvement techniques Observations:

Most TCP variants are NOT sufficient.A new transport layer protocol is needed.

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Overview of Results The best TCP variants:

TCP-Westwood and TCP-Jersey seem the best. Both protocols estimate bandwidth more accurately.

TCP mechanisms: Feedback from intermediate nodes leads to big gains.

The best non-TCP approaches: Ad-hoc Transport Protocol (ATP) seems to address most issues

Non-window based: estimates achievable rate periodically Split-TCP: promising new way of looking at transport layer

Dynamically buffer packets mid-path Key: Separation of congestion control from reliability.

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Roadmap

Overview of TCP The problems of TCP over MANETs Overview of best transport protocols In depth

Specific problems of TCP over MANETs Details of major TCP variants Discussion - other efforts

Conclusion

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Overview of TCP concepts Conventional TCP: Tahoe, Reno, New-Reno Sending rate is controlled by

Congestion window (cwnd): limits the # of packets in flight

Slow-start threshold (ssthresh): when CA start

Loss detection 3 duplicate ACKs (faster, more efficient) Retransmission timer expires (slower, less

efficient) Overview of congestion control mechanisms

Slow-start phase: cwnd start from 1 and increase exponentially

Congestion avoidance (CA): increase linearly Fast retransmit and fast recovery: Trigger by

3 duplicate ACKs

Overview

Slow-start Congestionavoidance

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What is different in MANETs?1. Mobility

Route stability and availability

1. High bit error rate Packets can be lost due to “noise”

1. Unpredictability/Variability Difficult to estimate time-out, RTT, bandwidth

1. Contention: packets compete for airtime Intra-flow and inter-flow contentions

1. Long connections have poor performance More than 4 hops thruput drops dramatically

Overview

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Overview of the Best Protocols TCP-Westwood

Estimate bandwidth to alleviate the effect of wireless errors. TCP-Jersey

Estimate bandwidth to alleviate the effect of wireless errors. Congestion warning assists the determination of packet loss

due to wireless error from congestion. ATP

Rate based transmission, periodic rate feedback, no timeout concept, reliability provided by SACK.

Split-TCP Separating congestion control from reliability. Dropped packets are recovered from the most recent proxy

instead of the source.

Overview

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Why does TCP fail in MANETs?Specific problems are identified:1. TCP misinterprets route failures as congestion2. TCP misinterprets wireless errors as congestion

3. Intra-flow and inter-flow contention reduce throughput and fairness

4. Delay spike causes TCP to invoke unnecessary retransmissions

RTO too small unnecessary retransmissions.

1. Inefficiency due to the loss of retransmitted packet When retransmitted packet is lost timer expires performance drops

Overview

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Roadmap

Overview of TCP The problems of TCP over MANETs Overview of best transport protocols In depth

Specific problems of TCP over MANETs Details of major TCP variants Discussion - other efforts

Conclusion

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Specific problems of TCP over MANETs

TCP misinterprets route failures as congestionEffects: Reduce sending rate Buffered packets (Data and ACKs) at

intermediate nodes are dropped.Sender encounters timeout.

Under prolonged disconnection, a series of timeouts may be encountered.

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TCP misinterprets wireless errors as congestion Effects: Incorrect execution of congestion control

Performance drops. Wireless channel is error-prone compared to wireline

Fading, interference, noise

Specific problems of TCP over MANETs

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Intra-flow and inter-flow contention Effects: Increased delay, unpredictability, and

unfairness. Inter-flow contention: contention of nearby flows. Intra-flow contention: between packets of the same

flow (e.g. forward data and reverse ACKs). Wireline: only packet on same link “compete”

Data stream

ACKs stream

Specific problems of TCP over MANETs

Two nearby flows

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Delay spike causes TCP to invoke unnecessary retransmissionsEffects: Performance drops and many

unnecessary retransmissions. [Ludwig & Katz]

Variability: Spikes are not uncommon hereSpikes throw off parameter estimation and

tuning RTO, window size, slow-start threshold

Specific problems of TCP over MANETs

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Inefficiency due to the loss of retransmitted packetEffects: Performance drops significantly

under high loss environment (e.g. MANETs).Losing a retransmitted packet hurts

TCP can recover from one loss (fast retransmission)

Wired networks: packet loss rate is low.Here, high packet loss makes the problem

significant

Specific problems of TCP over MANETs

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Classification of Transport protocols TCP variants try to improve the performance

by the following ways: Estimating the available bandwidth Determining route failure and wireless error Reducing contention Detecting spurious retransmission Exploiting buffering capability

New approaches: Non TCP variants Use rate based instead of window based approach Enable dynamic buffering (split TCP)

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Non-TCP based approaches

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A Rate-based Transport Layer Protocol Ad-hoc Transport Protocol (ATP) [Sundaresan et. al.]

Feedback from intermediate nodes path failure, queueing delay, periodic feedback on rate

Rate based transmission Entirely rate-controlled. (no window concept) Evenly distribute transmissions over time. (reduce burstniess)

Decoupling of congestion control and reliability Does not require the arrival of ACKs to clock out segment. Does not employ cumulative ACKs but solely relies on periodic SACK (with

20 SACK blocks) to identify losses. Pro: 1) Estimate rate accurately. 2) Reduce traffic on the reverse path. 3)

Recover more than one lost segment at a time. Con: 1) Incompatibility problem. 2) Require the assistance from the

intermediate nodes. 3) Fastest possible time to detect and recover packet lost is 1 second.

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Split-TCP: a new approach Split-TCP: work done at UCR [Kopparty et. al.]

Setup proxies along the connection many short TCP connections. Congestion control and reliability are separated. Proxies buffer packets from the previous proxy or the source. Any dropped packets are recovered from the most recent proxy but

not from the source. Pro: Enhance parallelism. Reduce bandwidth consumption on

retransmission. Con: Optimal frequency of proxy placement is not clear.

R SP P P

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Other Transport Layer EffortsCan FAST TCP and XCP work well over MANETs? Do not seem suitable for MANETs.

Basic idea: React faster to change. Fast TCP [Jin et. al.]

Determine equilibrium by queuing delay and loss information. cwnd far away from equilibrium? Rapid (Large) change. cwnd approach equilibrium? Small change.

XCP [Katabi et. al.] Explicit congestion signaling.

Intermediate nodes estimate spare bandwidth and generate feedback to the sender.

Neither protocol can deal with mobility. Mobility and route changes will throw off calculations.