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CS 3516: Advanced Computer Networks

Prof. Yanhua Li

Welcome to

Time: 9:00am –9:50am M, T, R, and FLocation: Fuller 320

Fall 2017 A-term

Some slides are originally from the course materials of the textbook “Computer Networking: A Top Down Approach”, 7th edition, by Jim Kurose, Keith Ross, Addison-Wesley March 2016. Copyright 1996-2017 J.F Kurose and K.W. Ross, All Rights Reserved.

Introduction

Lab assignment 1We are grading it.

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Quiz 2

Quiz 1Grades are available in Canvas.

Friday tomorrow

Introduction

Chapter 1: roadmap1.4 network performance in packet-switched networksdelaylossthroughput

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Introduction

Four sources of packet delay

dproc: nodal processing§ check bit errors§ determine output link§ typically < micro-sec

A

B

propagation

transmission

nodalprocessing queueing

dqueue: queueing delay§ time waiting at output link

for transmission § depends on congestion

level of router§ micro-sec to milli-sec

dnodal = dproc + dqueue + dtrans + dprop

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Introduction

dtrans: transmission delay:§ L: packet length (bits) § R: link bandwidth (bps)§ dtrans = L/R

dprop: propagation delay:§ d: length of physical link§ s: propagation speed in medium

(~2x108 m/sec)§ dprop = d/s

dtrans and dpropvery different

Four sources of packet delay

propagation

nodalprocessing queueing

dnodal = dproc + dqueue + dtrans + dprop

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A

B

transmission

When a packet can be transmitted on a link?v 1) no other pkt transmitted on the link (in practice)v 2) no other pkt preceding it in the queue

Propagation constant bRatio of propagation delay vs. packet transmission time

Wireless LAN 0.00004. Ethernet: 0.01

Source: Slides of Prof Ivan Marsic with Rutgers University

Introduction

Chapter 1: roadmap1.4 network performance in packet-switched networks

delaylossthroughput

1.5 protocol layers, service models2.1 Overview of application layer

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Introduction

v R: link bandwidth (bps)v L: packet length (bits)v a: average packet arrival

rate (pkt/s)

traffic intensity = La/R

v La/R: Traffic Intensity (pkts)v La/R ~ 0: avg. queueing delay smallv La/R -> 1: avg. queueing delay largev La/R > 1: more “work” arriving

than can be serviced, average delay infinite!

aver

age

que

uein

g de

lay

La/R ~ 0

Queueing delay (revisited)

La/R -> 11-8

Introduction

Nodal delay:

Ten packets are sent from A to B:Assumption: the time at which the 2nd packet is received at the first

router is equal to the time at which the 1st packet is received at the second router.

End-to-end delay

dnodal = dtrans + dprop = 2ms

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A

B

Dend-to-end, 10 = 3*dnodal + 9 * dnodal = 24 ms

Nodal delay: single packet end-to-end delay 6ms.

Nodal delay: ten packet end-to-end delay.Dend-to-end, single = 3*dnodal = 6 ms

Introduction

Chapter 1: roadmap1.4 network performance in packet-switched networks

delaylossthroughput

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Introduction

Packet lossv queue (aka buffer) preceding link in buffer has finite

capacityv packet arriving to full queue dropped (aka lost)v lost packet may be retransmitted by previous node,

by source end system, or not at all

A

B

packet being transmitted

packet arriving tofull buffer is lost

buffer (waiting area)

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Introduction

Chapter 1: roadmap1.4 network performance in packet-switched networks

delaylossthroughput

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Introduction

Throughput (an end-to-end measure)

v throughput: rate (bits/time unit) at which bits transferred between sender/receiver§ instantaneous: rate at given point in time§ average: rate over longer period of time

server, withfile of F bits

to send to client

link capacityRs bits/sec

link capacityRc bits/sec

server sends bits (fluid) into pipe

pipe that can carryfluid at rateRs bits/sec)

pipe that can carryfluid at rateRc bits/sec)

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Introduction

Throughput (more)

v Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

v Rs > Rc What is average end-end throughput?

link on end-end path that constrains end-end throughputbottleneck link

Rs bits/sec Rc bits/sec

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Rs

Rc

min(Rc,Rs)

Introduction

Throughput

R1=1Mbps

v per-connection end-end throughput:

max(R1,R2,R3)=R3=3Mbps

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R2=2Mbps

R3=3Mbps

Introduction

Throughput: Internet scenario

10 connections (fairly) share backbone bottleneck link R bits/sec

Rs

RsRs=2Mbps

Rc

Rc

Rc=1Mbps

R=5Mbps

v per-connection end-end throughput:

min(Rc,Rs,R/10)=500kbps

v in practice: Rc or Rs is often bottleneck

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Introduction

Chapter 1: roadmap1.4 delay, loss, throughput in networks1.5 protocol layers, service models

protocol layers, service models

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Introduction

Protocol “layers”Networks are complex,with many “pieces”:

§ hosts§ routers§ links of various

media§ applications§ protocols§ hardware,

software

Question:is there any hope of

organizing network processes in a structured way?

…. or at least our discussion of networks?

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Introduction

Organization of air travel

v a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

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Introduction

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Layering of airline functionality

layers: each layer implements a service§ via its own internal-layer actions§ relying on services provided by layer below

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Introduction

Why layering?dealing with complex systems:v Structured way to discuss the of complex system

§ layered reference model for discussionv modularization eases maintenance, updating of

system§ change of implementation of layer’s service

transparent to rest of system§ e.g., change in gate procedure doesn’t affect rest of

systemv layering considered harmful?

§ Duplicated functionalities at different layers, like error check

§ Functionality at one layer needs info from other layers1-21

Introduction

Internet protocol stackv application: supporting network

applications§ FTP, SMTP, HTTP, DNS

v transport: process-process data transfer§ TCP, UDP

v network: routing of datagrams from source to destination§ IP, routing protocols

v link: data transfer between neighboring network elements§ Ethernet, 802.11 (WiFi), PPP

v physical: bits “on the wire”

application

transport

network

link

physical

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Introduction

ISO/OSI reference model

v presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions

v session: synchronization, checkpointing, recovery of data exchange

v Internet stack “missing” these layers!§ these services, if needed, must be

implemented in application§ needed?

applicationpresentation

sessiontransportnetwork

linkphysical

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Introduction

sourceapplicationtransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destinationapplicationtransportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage M

Ht M

Hn

frame

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bits

Introduction

Introduction: summary

covered a “ton” of material!v S1: Internet overviewv S1: what’s a protocol?v S2: network edge, core, access

network§ packet-switching versus

circuit-switching§ Internet structure

v S3: Socket programmingv S4: performance: loss, delay,

throughputv S5: layering, service models

you now have:v context, overview, “feel”

of networkingv more depth, detail to

follow!

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Course Progression

v Week 1-2: Overview v Week 2-4: Application Layer Protocols

§ P2P, HTTP, SMTP, DNSv Week 4-5: Transport Layer Protocols

§ UDP and TCPv Week 6: IP, Routing Protocolsv Week 7: Link Layer Protocolsv Week 8: Wireless & Data Center Networkingv Slides for the lecture will be posted on the

website

Introduction

Questions?

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