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Setting up and Operating a PERT

Technical Session 1

Training Workbook

Technical Session 1 Technical Session 1 Training Workbook

Issue 1.0

© DANTE 2007

All trademarks are acknowledged.

Every effort has been made to ensure that the information contained in this document is true and correct at the time of going to press. However, the networks, systems, products, processes, specifications and content in general described in this document are subject to continuous development and

DANTE is entitled to change them at any time and to expand on them. DANTE cannot accept liability for any loss or damage of any nature whatsoever arising or resulting from the use of or reliance on information or particulars in this document.

All names and other data used in examples are fictitious.

The information contained in this document is of a general nature. Should you require further advice for your particular business requirements, please refer to the contact details below.

No part of this document may be reproduced by any means, other than with the express written permission of the copyright holder.

DANTE refers to Delivery of Advanced Network Technology to Europe Limited or any of its wholly owned subsidiaries.

Table of Contents

Table of Contents...................................................................................................3

Table of Slides........................................................................................................4

Introduction.............................................................................................................6

Aim of the Workbook......................................................................................7

Course Objectives..........................................................................................8

Course Outline................................................................................................9

Module 6 – Network Performance and User Expectations..................................10

Module 7 – Bulk Transfers Under TCP................................................................34Case Study: Finding out how to Tune Your Laptop’s TCP Stack.............................46

Case Study: Explore Performance Over two Contrasting Paths...............................75

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Table of SlidesWhat Factors Shape User Expectations?............................................................11

Users’ Perception of Performance.......................................................................12

The ‘Wizard Gap’..................................................................................................13

What Factors Really Shape Performance?..........................................................14

One-Way Delay (1)...............................................................................................15

One-Way Delay (2)...............................................................................................16

One-Way Delay (3)...............................................................................................17

Improving Delay...................................................................................................18

Round Trip Time (1).............................................................................................19

Round Trip Time (2).............................................................................................20

Round Trip Time (3).............................................................................................21

Delay Variation: An Example................................................................................22

Delay Variation: Definition and Implications.........................................................23

Packet Reordering (1)..........................................................................................24

Packet Reordering (2)..........................................................................................25

Packet Loss (1)....................................................................................................26

Packet Loss (2)....................................................................................................27

Packet Loss (3)....................................................................................................28

Packet Loss (4)....................................................................................................29

Packet Loss (5)....................................................................................................30

Maximum Transmission Unit (1)...........................................................................31



Overview of Bulk Transfers..................................................................................35

Window-Based Transmission...............................................................................36

How Does Window-Based Transmission Work?..................................................37

The Sliding Window..............................................................................................38

Window Size and Throughput..............................................................................39

Links, Paths and Pipes.........................................................................................40

Window Size Calculation......................................................................................41

How is the Window Size Determined?.................................................................42

Window Size and Performance............................................................................43

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Tuning Your Host for Maximum Transmission Rates (1).....................................44

Tuning Your Host for Maximum Transmission Rates (2).....................................45

The Goals of Early TCP (1)..................................................................................47

The Goals of Early TCP (2)..................................................................................48

‘Congestion Collapse’ in the Late 1980s..............................................................49

Why did Congestion Collapse Occur?.................................................................50

Why did Congestion Collapse Occur? (2)............................................................51

Introduction of Congestion Controls.....................................................................52

Avoiding Congestion: the Congestion Window....................................................53

Slow Start.............................................................................................................54

Congestion Collapse Fixed..................................................................................55

Digression: Self-Clocking.....................................................................................56

Congestion Avoidance and the Slow Start Threshold..........................................57

Slow start, the Slow Start Threshold and Congestion Avoidance: an Example...58

Dealing with Congestion: Principles.....................................................................59

TCP’s Response to Congestion...........................................................................60

Fast Retransmit and Fast Recovery.....................................................................61

The Window Closes Aggressively in Response to Retransmission Timeout.......62

The Window Closes Less in Response to Duplicate ACKs.................................63

Problems with TCP Congestion Control...............................................................64

Trouble With the Elephants (1).............................................................................65

Trouble With the Elephants (2).............................................................................66

High-Speed TCP Variants....................................................................................67

Explicit Congestion Control Protocols (1).............................................................68

Explicit Congestion Control Protocols (2).............................................................69

Implicit Congestion Control Protocols (1).............................................................70

Implicit Congestion Control Protocols (2).............................................................71

Network Tuning for Better Congestion Control (1)...............................................72



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Technical Session 1: Training WorkbookModule 7 – Bulk Transfers Under TCP

Introduction

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Aim of the Workbook

This workbook is designed for use in conjunction with the Technical Session 1 workshop.

The course is structured so that the facilitator presents functionality using slides. Where relevant and possible, the participants then have the opportunity to complete exercises to consolidate their learning. A training environment has been set up with training data provided, allowing participants to practice in a safe environment.

The aim of the workbook is to be a working document, reinforcing the information presented by the facilitator. Each delegate will have a copy of this workbook, which can be taken away and can be referred to at a later date. It is not designed to provide a detailed explanation of all aspects of a technology or application.

For further information about GÉANT2 training courses, mail [email protected].

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Course Objectives

By the end of this course, you will be able to:

Describe the rationale for and the structure and ethos of a federated PERT.

Set up local PERTs within a federated PERT structure.

Describe the methodologies used to investigate performance issues.

Use a variety of tools to investigate performance issues.

You will also have the opportunity to provide feedback about the evolution of the PERT and its systems.

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Course Outline

First Technical Session

Module 6 – Network Performance and User Expectations

Module 7 – Bulk Transfers Under TCP

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Module 6 – Network Performance and User Expectations

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What Factors Shape User Expectations?

Notes:

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Users’ Perception of Performance

Notes:

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The ‘Wizard Gap’

Notes:

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What Factors Really Shape Performance?

Notes:

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One-Way Delay (1)

Notes:

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One-Way Delay (2)

Notes:

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One-Way Delay (3)

Notes:

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Improving Delay

Notes:

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Round Trip Time (1)

Notes:

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Round Trip Time (2)

Notes:

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Round Trip Time (3)

Notes:

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Delay Variation: An Example

Notes:

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Delay Variation: Definition and Implications

Notes:

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Packet Reordering (1)

Notes:

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Packet Reordering (2)

Notes:

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Packet Loss (1)

Notes:

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Packet Loss (2)

Notes:

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Packet Loss (3)

Notes:

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Packet Loss (4)

Notes:

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Packet Loss (5)

Notes:

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Maximum Transmission Unit (1)

Notes:

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Notes:

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Notes:

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Module 7 – Bulk Transfers Under TCP

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Overview of Bulk Transfers

Notes:

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Window-Based Transmission

Notes:

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How Does Window-Based Transmission Work?

Notes:

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The Sliding Window

Notes:

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Window Size and Throughput

Notes:

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Links, Paths and Pipes

Notes:

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Window Size Calculation

Notes:

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How is the Window Size Determined?

Notes:

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Window Size and Performance

Notes:

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Tuning Your Host for Maximum Transmission Rates (1)

Notes:

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Tuning Your Host for Maximum Transmission Rates (2)

Notes:

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Case Study: Finding out how to Tune Your Laptop’s TCP Stack

Firstly, check your computer’s current configuration.

Examine the information in the PERT Knowledge Base (PERT KB) about your operating system. (see http://kb.pert.geant2.net/PERTKB/EndSystemTuning).

Using this information, find out how much throughput you can expect on a path with 100ms RTT in the best case (transmit and receive).

How would you tune your laptop to sustain 100Mbps at 120ms RTT (send and receive)?

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http://kb.pert.geant2.net/PERTKB/EndSystemTuning


The Goals of Early TCP (1)

Notes:

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The Goals of Early TCP (2)

Notes:

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‘Congestion Collapse’ in the Late 1980s

Notes:

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Why did Congestion Collapse Occur?

Notes:

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Why did Congestion Collapse Occur? (2)

Notes:

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Introduction of Congestion Controls

Notes:

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Avoiding Congestion: the Congestion Window

Notes:

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Case Study: Explore Performance Over two Contrasting Paths

1a) Assume a path with constant RTT and constant packet loss rate p << 1 with periodic loss, i.e. every sequence of 1/p packets of size MSS is followed by one lost packet. Calculate the average throughput during congestion avoidance if the receiver acknowledges every packet.

Hint: the congestion window oscillates between the maximum value W and W/2 in a "sawtooth" pattern with period W/2 RTT. The throughput is given by T = data per cycle/time of cycle and can be expressed as a function of p, RTT and MSS.

Solution: T = MSS/RTT * sqrt(3/(2*p))

1b) Compare the result to that of a window-limited transfer T = window/RTT

2) Give an intuitive explanation why a short path can tolerate much higher loss rates than a long path.

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