course overview

Chapter 1, slide: 1

Spring 2012ECE/CS 372 Introduction to Computer

Networks

Lecture 1

School of Electrical Engineering and Computer ScienceOregon State University

Course Overview

Chapter 1, slide: 2

Course website http://web.engr.oregonstate.edu/~qassim/index_files/

ComputerNetworks.htm

Please write down this URL—all course material and information will be provided thru this site

Lectures Everyday 1-1:50pm

Instructor Yousef Qassim ([email protected])

Office hours: TR 2:30-3:20pm @KEC lounge

Lecture/Office/Lab Hours

Chapter 1, slide: 3

Lab AssistantLocation: Dearborn 205Lab hours: to help you with your labs

205 DEARInformation can be found in course’s website

LabLocation: Dearborn 205Access code: will be written on board

Lecture/Office/Lab Hours

Chapter 1, slide: 4

Prerequisite: CS or ECE 271 or an equivalent course Basic Linux familiarity

Textbook

Prerequisite/Textbook

Textbook is RequiredComputer Networking: A Top-Down Approach Featuring the Internet, 6th Edition, Games F. Kurose, Keith W. Ross

Chapter 1, slide: 5

Grading Policy Assignments: 15%

Each student must hand in one copy 5 assignments: approx. 1 every two weeks

Labs: 15% Each student must hand in one copy 5 labs: approx. 1 every two weeks

One midterm exam: 30% Final exam: 40%

Chapter 1, slide: 6

Topics To Be Covered Architecture of the Internet, and network

protocols

Delay analysis

Packet-switching and circuit-switching

Congestion and flow control: TCP

Routing algorithms: IP and datagram

Data link layers and Ethernet: ARP, CSMA/CD

Medium access control and local area networks

Chapter 1, slide: 7

Lectures & assignments

Objective Deep understanding of basic and fundamental

networking concepts, architectures, and philosophies

IMPORTANT: this course is NOT about setting up your router at home, or writing a twitter program!!

Approach: how to do well in this course Easy: attend ALL lectures and do ALL assignments Do your assignments individually Do NOT miss any Bonus Quiz (i.e., do not miss class) Some hw problems will be solved in class: this gives

you the opportunity to clarify things further

Chapter 1, slide: 8

Labs

Objective Understand how Internet protocols work Force network protocols to perform certain

actions Observe and analyze protocols’ behavior

Approach Software tool: Wireshark

already installed in Lab DEAR 205 To run, type: sudo wireshark then enter your eecs

psswd Allows you to sniff and analyze traffic

sent/received from/by your end system: real measurement of Internet traffic

Chapter 1, slide: 9

Chapter 1: Introduction

Our goal: learn basic network

terminologies more depth, detail

later in course approach:

use Internet as example

Acknowledgement: slides drawn heavily from Kurose & Ross

Chapter 1, slide: 10

Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Internet structure and ISPs5 Protocol layers, service models

6 Delay & loss in packet-switched networks


What’s the Internet: a “service” view communication

infrastructure enables distributed apps: Enables apps to

communicate Web, email, games, e-

commerce, file sharing

communication services provided to apps: Offers services


What’s the Internet: “nuts and bolts” view

millions of connected computing devices: called hosts or end systems e.g., Laptops, workstations running network apps

routers & switches: forward packets (chunks of

data)

communication links e.g., fiber, copper, radio,

satellite

local ISP

companynetwork

regional ISP

router workstation

servermobile


What’s the Internet: “nuts and bolts” view Internet standards

IETF (Internet Eng. Task Force)

• RFC: Request for comments

IEEE: for links/hardware E.g., Ethernet

network protocols control sending/receiving

of messages e.g., TCP, IP, HTTP, FTP,

PPP

local ISP

companynetwork

regional ISP

router workstation

servermobile


What’s a protocol?a human protocol and a computer network protocol:

Hi

Hi

Got thetime?

2:00

TCP connection request

TCP connectionresponseGet http://www.awl.com/kurose-ross

<file>time


What’s a protocol?human protocols: “what’s the time?” “I have a question” introductions

… specific msgs sent… specific actions

taken when msgs received, or other events

network protocols: machines rather than

humans all communication

activity in Internet governed by protocols

protocols define (1) format, order of msgs

sent and received among network entities, and (2) actions taken on

msg transmission, receipt


Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Internet structure and ISPs 5 Protocol layers, service models



A closer look at network structure: network edge:

applications and hosts network core:

routers network of networks

access networks, physical media: communication links


The network edge: service models

end systems (hosts): run application programs e.g. Web, email at “edge of network”

client/server model client host requests, receives

service from always-on server e.g. Web browser/server; email

client/server

peer-to-peer model: minimal (or no) use of

dedicated servers e.g. Skype, BitTorrent, KaZaA


Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Network access and physical media5 Internet structure and ISPs 6 Protocol layers, service models



The Network Core

mesh of interconnected routers

the fundamental question: how is data transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”


Network Core: Circuit Switching

End-end resources reserved for “call”

dedicated resources: no sharing

call setup required circuit-like

(guaranteed) performance

same path for all chunks



network resources (e.g., bandwidth) divided into “pieces”

allocated pieces per call

no sharingresource piece idle if not used by owning call



Two ways of dividing bandwidth into “pieces” frequency division time division


Circuit Switching: FDM and TDM

Freq. Division Multiplx. (FDM)

frequency

time

Time Division Multiplx. (TDM)

frequency

time

4 users

Example:


Network Core: Packet Switching

each end-to-end data stream is divided into packets no dedication/reservation: all streams share resources

no setup is required resources used as needed each packet uses full link bandwidth aggregate resource demand can exceed capacity no guarantee

A

B

C100 Mb/sEthernet

1.5 Mb/s


Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing.

Whereas in TDM, each host gets same slot (periodically)

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

Network Core: statistical multiplexing


A

B2 Mb/s

Circuit switching

B: has nopackets to send

• 2 circuits (use TDM)• A reserves 1 circuit• B reserves 1 circuit

A

B2 Mb/s

Packet switching

• statistical multiplex.• B uses full link since A is not using it

Packet switching versus circuit switching

Utilization = 50% only = 1 Mb/s

Utilization = 100% = 2 Mb/s



Packet-switching Circuit-switching

Resources sharing dedicated

Congestion may lead to it admission control

Overhead less overhead; more overhead;

no connection setup reserve resources 1st

Guarantee Best-effort provide guarantee

no guarantee good for multimedia


Numerical example

How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? The link’s transmission rate = 0.64 Mbps Each link uses TDM with 10 slots/sec 0.5 sec to establish end-to-end circuit

Let’s work it out! You have few minutes!

Solution: Bandwidth of circuit (in kbps)= .64x1000/10 = 64

kbps Time to send: 640 kbits/64 kbps + 0.5s = 10.5s


ECE/CS 372 – introduction to computer networks

Lecture 2

Announcements:

Please make sure to check the course’s website in a regular basishttp://web.engr.oregonstate.edu/~qassim/index_files/ComputerNetworks.htm

Lab 1 is due on Tuesday

ALL HW and LAB ASSIGNMENTS SHOULD BE HARD COPY, SOFT COPY IS NOT ACCEPTED


Announcements Lab

Location: Dearborn 205Access code: will be written on board

Approach: how to do well in this course Easy: attend ALL lectures and do ALL

assignments Do your assignments individually Some hw problems will be solved in class: this

gives you the opportunity to clarify things further




Packet-switching Circuit-switching

Resources sharing dedicated

Congestion may lead to it admission control

Overhead less overhead; more overhead;

no connection setup reserve resources 1st

Guarantee Best-effort provide guarantee

no guarantee good for multimedia



3 Mb/s link each user:

1 Mb/s when “active” active 1/3 of time

circuit-switching: 3 users

packet switching: With N=4 users, what are the chances that a user won’t get 1 Mb/s? I.e., what is the prob. that more than 3 (strictly) users are active?

With N=5 users, what are the chances that a user won’t get 1 Mb/s?

With N=6 users, what are the chances that a user won’t get 1 Mb/s?

Packet switching allows more users to use network!

N users

3 Mbps link


Packet switching versus circuit switchingBoard …



Lecture 3

Announcements:

HW1 is posted and is due Monday of the 3rd week in class.

Hard copy only, late HW are not accepted.

Announcement section is added to the website



Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Internet structure and ISPs5 Protocol layers, service models



Internet structure: network of networks

roughly hierarchical: tier 1, tier 2, and tier 3 at center: “tier-1” ISPs

e.g., MCI, Sprint, AT&T, Cable and Wireless, national/international coverage

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)


Tier-1 ISP: e.g., Sprint

Sprint US backbone network

Seattle

Atlanta

Chicago

Roachdale

Stockton

San Jose

Anaheim

Fort Worth

Orlando

Kansas City

CheyenneNew York

PennsaukenRelay

Wash. DC

Tacoma

DS3 (45 Mbps)OC3 (155 Mbps)OC12 (622 Mbps)OC48 (2.4 Gbps)



“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP



“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet



a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP


Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Internet structure and ISPs5 Protocol layers, service models6 Delay & loss in packet-switched

networks


Protocol “Layers”Networks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

Question: Is there any hope of an

organizing structure of network?


Organization of air travel

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


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


Why layering?

Dealing with complex systems:

Easing assignment of tasks identify relationship among pieces of

complex systems

Easing 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 system


Internet protocol stack application: supporting network

applications FTP, SMTP, HTTP

transport: process-process data transfer TCP, UDP

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

link: data transfer between neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical


sourceapplicatio

ntransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

application

transportnetwork

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


ISO/OSI Model: late 70’s

application

transport

network

link

physical

presentation

application

session

transport

network

data link

physical

7-layer ISO/OSI model(OSI: open system interconnections)

5-layer Internet Protocol Stack


Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Internet structure and ISPs 5 Protocol layers, service models6 Delay & loss in packet-switched

networks


Sources of packet delay

1. processing: check bit errors determine output link

2. queueing time waiting at output

link for transmission depends on

congestion level of router

A

Bnodal

processing queueing


3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) trans. delay = L/R

4. Propagation delay: d = length of physical link s = propagation speed in

medium (~2x108 m/sec) propagation delay = d/s

Note: s and R are very different quantities!

A

B

propagation

transmission

nodalprocessing queueing

Sources of packet delay


Caravan analogy

Cars run at 100 km/hr (speed of propagation)

Booth takes 12 sec to service a car (transmission time)

Car ~ bit; caravan ~ packet

Q: How long until caravan is lined up before 2nd toll booth?

Time to “push” entire caravan through toll booth = 12*10 = 120 sec = 2 mns

Time for last car to propagate from 1st to 2nd toll both: =100km/(100km/hr)= 1 hr

A: 1 hr 2 minutes

toll booth

toll booth

ten-car caravan

100 km

100 km


Caravan analogy (more)

Cars now “propagate” at 1000 km/hr

Toll booth now takes 1 min to service a car

Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?

Yes! After 7 min, 1st car at 2nd booth and 8th car still at 1st booth.

1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!

toll booth

toll booth

ten-car caravan

100 km

100 km


Exercise 1

Host A Host Btrans. rate R = 1 Mbps

distance = 1 km, speed = 2x108m/s

Packet length = L bits

Question: Which bit is being transmitted at the time the first bit

arrives at Host B for

Answer:First bit arrives after 1/R + d/s = 1/106 + 103/(2x108) = 10-6 + 5x10-6 = 6x10-6 =

6 µsec

After 6 µsec6 bits are already transmitted; so 7th bit is being transmitted


Exercise 2Transmission vs. propagation

Host A Host Btrans. rate R = ?

distance = 2 km, speed = 2x108m/s

L=100Bytes

Question: At what rate (bandwidth) R would the propagation delay

equal the transmission delay?

Answer: Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec Transmission delay = 100x8 (bits)/R Prop. delay = trans. delay => R=105x100x8 = 80 Mbps


Exercise 3Voice over IP

Host A Host Btrans. rate R = 1Mbps

delay_prop = 2mseca=64Kbps

L=48 Bytes

Host A converts analog to digital at a=64Kbps groups bits into L=48Byte packets sends packet to Host B as soon it gathers a packet

Host B As soon as it receives the whole pckt, it converts it to analog

Question: How much time elapses from the 1st bit of 1st packet is

created until the last bit of the 1st packet arrives at Host B?


Exercise 3Voice over IP

Host A Host Btrans. rate R = 1Mbps

delay_prop = 2msec

Answer: Time to gather 1st pkt: 48x8 (bits)/64x1000 (b/s) = 6 msec

Time to push 1st pkt to link: 48x8 (bits)/1x106 (b/s) = 0.384 msec

Time to propagate: 2 msec

Total delay = 6 + 0.384 + 2 = 8.384 msec

a=64Kbps

L=48 Bytes



Lecture 4

Announcements: Lab 1 is due Tuesday 2nd week

HW1 is due Monday 3rd week

No class on Friday, Friday is a lab hour 1st week 1-2pm, later on 2:30-3:20pmYou don’t have to show up and you can come to me with your lab or class related questions



Nodal delay

dproc = processing delay typically a few microsecs or less

dqueue = queuing delay depends on congestion

dtrans = transmission delay = L/R, significant for low-speed links

dprop = propagation delay a few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd


Queueing delay (more insight)

Every second: aL bits arrive to queue Every second: R bits leave the router Question: what happens if aL > R ? Answer: queue will fill up, and packets will get

dropped!!

aL/R is called traffic intensity

queuePacket arrival rate= a packets/sec

Link bandwidth = R bits/sec



Queueing delay: illustration

Arrival rate: a = 1/(L/R) = R/L (packet/second)

Traffic intensity = aL/R = (R/L) (L/R) = 1

Average queueing delay = 0(queue is initially empty)

queueLink bandwidth = R bits/sec

1 packet arrivesevery L/R seconds

Packet length L bits


Queueing delay: illustration

Arrival rate: a = N/(LN/R) = R/L packet/second

Traffic intensity = aL/R = (R/L) (L/R) = 1

Average queueing delay (queue is empty @ time 0) ?{0 + L/R + 2L/R + … + (N-1)L/R}/N = L/(RN){1+2+…+(N-1)} =L(N-1)/(2R)Note: traffic intensity is same as previous scenario, but queueing delay is different

queueLink bandwidth = R bits/sec

N packet arrive simultaneouslyevery LN/R seconds

Packet length L bits


Queueing delay: behavior

La/R ~ 0: avg. queuing delay small

La/R -> 1: delays become large La/R > 1: more “work” than can

be serviced, average delay infinite!

(this is when a is random!)

queuePacket arrival rate= a packets/sec

Link bandwidth = R bits/sec



Packet-switching: store-and-forward

Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps

delay = 3L/R (assuming zero propagation delay)more on this next…

R R RL

Entire packet must arrive at router before it can be transmitted on next link: store and forward


Store-and-forward: illustration

delay (one packet only) = L/R + d/s

R

L d

distance = d meters; speed of propagation = s m/sec

transmission rate of link = R bits/s

delay (one packet only) = L/R + ½d/s + L/R + ½d/s = 2L/R + d/s

Example:d/s = 0.5 secL = 10 MbitsR = 1 Mbpsdelay = 10.5 sec

Example:d/s = 0.5 secL = 10 MbitsR = 1 Mbpsdelay = 20.5 sec

R R

Ld/2 d/2


Store-and-forward & queuing delay

Case 1: Assume R1 < R2

distance = d meters; speed of propagation = s m/sec

transmission rate of link = R1 and R2 bits/s Consider sending two packets A and B back to

back

R1 R2

Ld

Case 2: Assume R1 > R2

Q: is there a queuing delay? how much is this delay?Answer (queue is empty initially):

Time for last bit of 2nd pkt to arrive at router: d1= L/R1 + L/R1 + d/(2s)Time for last bit of 1st pkt to leave router: d2= L/R1 + d/(2s) + L/R2Queueing delay = d2 – d1 = L/R2 – L/R1 if positive, otherwise 0. Hence:when R1 < R2, queueing delay = d2 – d1 = 0when R1 > R2, queueing delay = d2 – d1 = L/R2 – L/R1


Throughput analysis

Suppose: Host A has huge file of size F bits to send to Host B File is split into N packets, each of length L bits (i.e., N=F/L) Ignore propagation delay for now

R R RLHost A Host B

Question 1: how long it takes to send the file?A: (N+2)L/R = (F+2L)/R

Question 2: what is the average throughput achieved when sending the file?A: NL/[(N+2)L/R]=NR/(N+2) = FR/(F+2L) = R/(1+2L/F)

Note: throughput = number of total bits sent / total time taken


Throughput analysis

Suppose: Host A has huge file of size F bits to send to Host B File is split into N packets, each of length L bits (i.e., N=F/L) Do NOT ignore propagation delay (assume prop. speed = s m/s)

R R RLHost A Host B

Question 1: how long it takes to send the file?A: (N+2)L/R + d/s = (F+2L)/R + d/s

Question 2: what is the average throughput achieved when sending the file?A: NL/[(N+2)L/R +d/s]=FR/[(N+2)L + dR/s] = FR/[F+2L+dR/s]

d/3 d/3d/3


Introduction: Summary

Covered a “ton” of material! Internet overview Network protocol Network edge, core, access network Packet-switching versus circuit-switching Internet/ISP structure layering and service models performance: delay and throughput

analysis

course overview

Documents

distributed apps

lab assistantlocation

lab dear

kurose rosschapter

course website http

ch student

copy5 assignments

site lectures