Week 1 - WebCMS3

Introduction

Introduction to Computer Networks (chapter 1 of text)

Computer Networks and Applications

Week 1

COMP 3331/COMP 9331

Reading Guide: Chapter 1, Sections 1.1 - 1.4

Introduction

Acknowledgment

v  Majority of lecture slides are from the author’s lecture slide set §  Several Enhancements + additional material

1-2

Introduction

1. Introduction Goals: v  get “feel” and terminology v  defer depth and detail to later in course v  understand concepts using the Internet as example

1-3

Introduction

1. Introduction: roadmap 1.1 what is the Internet? 1.2 network edge

§  end systems, access networks, links

1.3 network core §  packet switching, circuit switching, network structure

1.4 delay, loss, throughput in networks 1.5 protocol layers 1.6 networks under attack: security 1.7 history

1-4

Hobbe’s Internet Timeline - http://www.zakon.org/robert/internet/timeline/

Introduction 1-5

Quiz: What is the Internet?

A. One single homogenous network B. An interconnection of different computer networks C. An infrastructure that provides services to networked applications D. Something else (be prepared to discuss)

Introduction 1-6

Computer)Networks,)Fall)2015 2

end$system

Windows-PCLinux-server MAC-laptop

car-navigator

heart-pacemaker

smartphone

iPad

Introduction 1-7


end$system switch (routers)

Introduction 1-8 Computer)Networks,)Fall)2015

phone-lines

4

end$system switch

link

fibers

cable-TV-lines

wireless-links

Introduction 1-9


end$system switch

link

Internet-Service-Provider

phone-company

cable-companyuniversity-net

Introduction 1-10


end$system switch

link

packet

path

Internet-Service-Provider

Introduction 1-11


instant-messaging

instant-messaging

facebook-server

firefox-accessing-facebook

world-of-warcraE-client

world-of-warcraE-server

Introduction 1-12


while'(...)'{''''message'='...;''''send'('message,'...');'}

while'(...)'{''''message'='receive'('...');'}

Alice

Bob

Introduction 1-13

Computer)Networks,)Fall)2015

while'(...)'{''''message'='receive'('...');'}

9

ApplicaGon-Programming-InterfaceAlice

while'(...)'{''''message'='...;''''send'('message,'...');'}

Bob

Introduction 1-14


Alice Bob

hello

hello

give-me-hHp://nal.epfl.ch

here:-...

Protocols – Language of Communication

Introduction

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

v millions of connected computing devices: §  hosts = end systems §  running network apps

v communication links §  fiber, copper, radio,

satellite §  transmission rate:

bandwidth

v Packet switches: forward packets (chunks of data) §  routers and switches

wired links

wireless links

router

mobile network

global ISP

regional ISP

home network

institutional network

smartphone

PC

server

wireless laptop

1-15

Self-study

Introduction

“Fun” internet appliances

IP picture frame http://www.ceiva.com/

Web-enabled toaster + weather forecaster

Smart Lightbulbs Internet refrigerator

Networked TV Set top Boxes

1-16

Tweet-a-watt: monitor energy use

Self-study

Introduction 1-17

http://thenextweb.com/insider/2012/12/09/the-future-of-the-internet-of-things/ http://gigaom.com/2013/03/05/the-future-of-the-internet-is-avatars-and-connected-services-video/ http://share.cisco.com/assets/images/Internet_of_Things_Infographic.jpg

Source: cisco Source: www.beyondplm.com

Self-study

Introduction

v  Internet: “network of networks” §  Interconnected ISPs

v  protocols control sending, receiving of msgs §  e.g., TCP, IP, HTTP, Skype, 802.11

v  Internet standards §  RFC: Request for comments §  IETF: Internet Engineering Task

Force

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

global ISP

regional ISP

home network


1-18

Self-study

What’s the Internet: a service view

v  Infrastructure that provides services to applications: §  Web, VoIP, email, games, e-

commerce, social nets, … v  provides programming

interface to apps §  hooks that allow sending

and receiving app programs to “connect” to Internet

§  provides service options, analogous to postal service

mobile network

global ISP

regional ISP

home network


Introduction 1-19

Self-study

Introduction

What’s a protocol?

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

… specific msgs sent … specific actions taken

when msgs received, or other events

network protocols: v  machines rather than

humans v  all communication activity

in Internet governed by protocols

protocols define format, order of msgs sent and received among network entities,

and actions taken on msg transmission, receipt

1-20

Self-study

Introduction

a human protocol and a computer network protocol:

Q: other human protocols?

Hi

Hi

Got the time? 2:00

TCP connection response

Get http://www.awl.com/kurose-ross

<file> time

TCP connection request


1-21

Self-study

Introduction

Silly example: Clay Tablet Carrier Pigeon Protocol (CTCP) v  Inscribe message on clay tablet v  Let dry in the sun v  Tie to leg of carrier pigeon v  Let pigeon go v  Viola – your message is sent !! v  Recipient catches pigeon and reads tablet Questions for later:

- Is it connectionless? - Is it reliable?

IP over avian carriers – RFC1149 http://en.wikipedia.org/wiki/IP_over_Avian_Carriers


1-22

Self-study

Introduction




1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history

1-23

Self-study

Introduction

A closer look at network structure:

v  network edge: §  hosts: clients and servers §  servers often in data

centers

v  access networks, physical media: wired, wireless communication links

v  network core: § interconnected routers § network of networks

mobile network

global ISP

regional ISP

home network


1-24

Self-study

Introduction

Access networks and physical media

Q: How to connect end systems to edge router?

v  residential access nets v  institutional access

networks (school, company)

v  mobile access networks

keep in mind: v  bandwidth (bits per second)

of access network? v  shared or dedicated?

1-25

Self-study

Introduction

Access net: digital subscriber line (DSL)

central office

ISP

telephone network

DSLAM

voice, data transmitted at different frequencies over

dedicated line to central office

v  use existing telephone line to central office DSLAM

§  data over DSL phone line goes to Internet §  voice over DSL phone line goes to telephone net

v  < 2.5 Mbps upstream transmission rate (typically < 1 Mbps) v  < 24 Mbps downstream transmission rate (typically < 10 Mbps)

DSL modem

splitter

DSL access multiplexer

1-26

Self-study

Introduction

Access net: cable network

cable modem

splitter

… cable headend

Channels

V I D E O

V I D E O

V I D E O

V I D E O

V I D E O

V I D E O

D A T A

D A T A

C O N T R O L

1 2 3 4 5 6 7 8 9

frequency division multiplexing: different channels transmitted in different frequency bands

1-27

Self-study

Introduction

data, TV transmitted at different frequencies over shared cable

distribution network

cable modem

splitter

… cable headend

CMTS

ISP

cable modem termination system

v  HFC: hybrid fiber coax §  asymmetric: up to 30Mbps downstream transmission rate, 2

Mbps upstream transmission rate v  network of cable, fiber attaches homes to ISP router

§  homes share access network to cable headend §  unlike DSL, which has dedicated access to central office

Access net: cable network

1-28

Self-study

Fiber to the home (FTTH)

v  Fully optical fiber path all the way to the home §  e.g., Verizon FIOS, Google, NBN §  ~30 Mbps to 1Gbps

v  Active (like switched Ethernet) or passive optical networking as shown below

Introduction 1-29

Self-study

Introduction

Access net: home network

to/from headend or central office

cable or DSL modem

router, firewall, NAT

wired Ethernet (100 Mbps)

wireless access point (54 Mbps)

wireless devices

often combined in single box

1-30

Self-study

Introduction

Enterprise access networks (Ethernet)

v  typically used in companies, universities, etc v  10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates v  today, end systems typically connect into Ethernet switch

Ethernet switch

institutional mail, web servers

institutional router

institutional link to ISP (Internet)

1-31

Self-study

Introduction

Wireless access networks v  shared wireless access network connects end system to router

§  via base station aka “access point”

wireless LANs: §  within building (100 ft) §  802.11b/g (WiFi): 11, 54 Mbps

transmission rate

wide-area wireless access §  provided by telco (cellular)

operator, 10’s km §  between 1 and 10 Mbps §  3G, 4G: LTE

to Internet

to Internet

1-32

Self-study

Host: sends packets of data

host sending function: v  takes application message v  breaks into smaller

chunks, known as packets, of length L bits

v  transmits packet into access network at transmission rate R §  link transmission rate,

aka link capacity, aka link bandwidth

R: link transmission rate host

1 2

two packets, L bits each

packet transmission

delay

time needed to transmit L-bit

packet into link

L (bits) R (bits/sec) = =

1-33

Self-study

Introduction

Physical media

v  bit: propagates between transmitter/receiver pairs

v  physical link: what lies between transmitter & receiver

v  guided media: §  signals propagate in solid

media: copper, fiber, coax v  unguided media:

§  signals propagate freely, e.g., radio

1-34

Self-study

Introduction

Physical media: twisted pair, coax, fiber

coaxial cable: v  two concentric copper

conductors v  broadband:

§  multiple channels on cable §  HFC

fiber optic cable: v  glass fiber carrying light

pulses, each pulse a bit v  high-speed operation:

§  high-speed point-to-point transmission (e.g., 10’s-100’s Gpbs transmission rate)

v  low error rate: §  repeaters spaced far apart §  immune to electromagnetic

noise

1-35

twisted pair (TP) v  two insulated copper

wires §  Category 5: 100 Mbps, 1

Gpbs Ethernet §  Category 6: 10Gbps

Self-study

Introduction

Physical media: radio

v  signal carried in electromagnetic spectrum, i.e., no physical “wire”

v  propagation environment

effects: §  reflection §  obstruction by objects §  interference

radio link types: v  terrestrial microwave

§  e.g. up to 45 Mbps channels

v  LAN (e.g., WiFi) §  11Mbps, 54 Mbps

v  wide-area (e.g., cellular) §  3G cellular: ~ few Mbps

v  satellite §  Kbps to 45Mbps channel (or

multiple smaller channels) §  270 msec end-end delay §  geosynchronous versus low

earth-orbitting (LEO)

1-36

Self-study

Introduction





1-37

Introduction

v  mesh of interconnected routers/switches

v  Two forms of switched networks: §  Circuit switching: used in

the legacy telephone networks

§  Packet switching: used in the Internet

The network core

1-38

Circuit switching

(1) Node A sends a reservation request (2) Interior switches establish a connection -- i.e., “circuit” (3) A starts sending data (4) A sends a “teardown circuit” message

Idea: source reserves network capacity along a path

A B 10Mb/s?

10Mb/s?

10Mb/s?

1-39 Introduction

Old Time Multiplexing

1-40 Introduction

Introduction

Circuit switching: FDM versus TDM

FDM

frequency

time TDM

frequency

time

4 users Example:

1-41

Information time

Timing in Circuit Switching

Circuit Establish

ment

Transfer

Circuit Tear-down 1-42 Introduction

Numerical example

v  How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? §  All links are 1.536 Mbps §  Each link uses TDM with 24 slots/sec §  500 msec to establish end-to-end circuit

Let’s work it out!

1/24*1.536Mb/s=64kb/s640,000/64kb/s=10s10s+500ms=10.5s

1-43 Introduction

Consider a circuit-switched network with N=100 users where each user is active with probability p=0.2 and when active, sends data at a rate R=1Mbps. How much capacity must the network be provisioned with?

A.  100 Mbps B.  20 Mbps C.  200 Mbps D.  50 Mbps E.  500 Mbps

Introduction 1-44

Quiz: Circuit Switching Capacity

v  Pros:

v  Cons:

1-45 Introduction

Quiz: What are the pros and cons of circuit switching?

Packet Switching

v  Data is sent as chunks of formatted bits (Packets) v  Packets consist of a “header” and “payload”*

01000111100010101001110100011001

1.  Internet Address 2. Age (TTL) 3. Checksum to protect header

Header Data header

payload 1-46 Introduction

Packet Switching

v  Data is sent as chunks of formatted bits (Packets) v  Packets consist of a “header” and “payload”*

§  payload is the data being carried §  header holds instructions to the network for how to

handle packet (think of the header as an API)

1-47 Introduction

Packet Switching

v  Data is sent as chunks of formatted bits (Packets) v  Packets consist of a “header” and “payload” v  Switches “forward” packets based on their headers

1-48 Introduction

Switches forward packets

EDINBURGH

OXFORD

GLASGOW

UCL

Destination Next Hop

GLASGOW 4

OXFORD 5

EDIN 2

UCL 3

Forwarding Table 111010010 EDI

N

switch#2

switch#5

switch#3

switch#4

1-49 Introduction

time

Timing in Packet Switching

payload hdr

What about the time to process the packet at the switch? •  We’ll assume it’s relatively negligible (mostly true)

1-50 Introduction

time

Timing in Packet Switching payloa

d hdr

Could the switch start transmitting as soon as it has processed the

header?

Introduction 1-51

time

Timing in Packet Switching payloa

d hdr

Could the switch start transmitting as soon as it has processed the

header?

Yes! This would be called a “cut through” switch

Introduction 1-52

time

Timing in Packet Switching

payload hdr

We will always assume a switch processes/forwards a packet after it has received it entirely.

This is called “store and forward” switching 1-53 Introduction

Packet Switching

v  Data is sent as chunks of formatted bits (Packets) v  Packets consist of a “header” and “payload” v  Switches “forward” packets based on their headers

1-54 Introduction

Packet Switching

v  Data is sent as chunks of formatted bits (Packets) v  Packets consist of a “header” and “payload” v  Switches “forward” packets based on their headers v  Each packet travels independently

§  no notion of packets belonging to a “circuit”

1-55 Introduction

Packet Switching

v  Data is sent as chunks of formatted bits (Packets) v  Packets consist of a “header” and “payload” v  Switches “forward” packets based on their headers v  Each packet travels independently v  No link resources are reserved in advance. Instead

packet switching leverages statistical multiplexing

1-56 Introduction

Multiplexing

Sharing makes things efficient (cost less) v  One airplane/train for 100 people v  One telephone for many calls v  One lecture theatre for many classes v  One computer for many tasks v  One network for many computers v  One datacenter many applications

1-57 Introduction

Data Rate 1

Data Rate 2

Data Rate 3

Three Flows with Bursty Traffic

Time

Time

Time Capacity

1-58 Introduction

Data Rate 1

Data Rate 2

Data Rate 3

When Each Flow Gets 1/3rd of Capacity

Time

Time

Time

Frequent Overloading

1-59 Introduction

When Flows Share Total Capacity

Time

Time

Time

No Overloading

Statistical multiplexing relies on the assumption that not all flows burst at the same time.

Very similar to insurance, and has same failure case

1-60 Introduction

Data Rate 1

Data Rate 2

Data Rate 3


Time

Time

Time Capacity

1-61 Introduction

Data Rate 1

Data Rate 2

Data Rate 3


Time

Time

Time Capacity

1-62 Introduction

Data Rate 1+2+3 >> Capacity


Time

Time Capacity

What do we do under overload? 1-63 Introduction

time à

BW à

pkt tx time

1-64 Introduction

Statistical multiplexing: pipe view

1-65 Introduction


No Overload

1-66 Introduction


Transient Overload Not such a rare event

Queue overload into Buffer

1-67 Introduction




1-68 Introduction





1-69 Introduction



1-70 Introduction




1-71 Introduction


Transient Overload Not a rare event! Buffer absorbs transient bursts


1-72 Introduction


What about persistent overload? Will eventually drop packets


1-73 Introduction


Introduction

Packet Switching: queueing delay, loss

A

B

C R = 100 Mb/s

R = 1.5 Mb/s D

E queue of packets waiting for output link

1-74

queuing and loss: v  If arrival rate (in bits) to link exceeds transmission rate of

link for a period of time: §  packets will queue, wait to be transmitted on link §  packets can be dropped (lost) if memory (buffer) fills up

v  Pros:

v  Cons:

1-75 Introduction

Quiz: What are the pros and cons of packet switching?

76

Packet switching versus circuit switching Packet-switching Circuit-switching

v  Resources sharing dedicated

v  Congestion may lead to it admission control

v  Overhead less overhead; more overhead; no connection setup reserve resources 1st

v  Guarantee Best-effort provide guarantee

no guarantee good for multimedia

Introduction

Introduction

v  great for bursty data §  resource sharing §  simpler, no call setup

v  excessive congestion possible: packet delay and loss §  protocols needed for reliable data transfer, congestion

control v  Q: How to provide circuit-like behavior?

§  bandwidth guarantees needed for audio/video apps §  still an unsolved problem (chapter 7)

is packet switching a “slam dunk winner?”

Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?

Packet switching versus circuit switching

1-77

Introduction

Packet switching versus circuit switching

example: §  1 Mb/s link §  each user:

•  100 kb/s when “active” •  active 10% of time

v circuit-switching: §  10 users

v packet switching: §  with 35 users, probability >

10 active at same time is less than .0004 *

packet switching allows more users to use network!

N users

1 Mbps link

Q: how did we get value 0.0004?

Q: what happens if > 35 users ?

…..

1-78

Further Resources: Problem 8 & 9, Chapter 1, Kurose and Ross, 6th Edition http://gaia.cs.umass.edu/kurose_ross/interactive/ps_versus_cs.php

Self-study

Introduction 79

v  Probability Basics: Bernoulli Trials §  Let p be the probability of some event A

•  Examples of A –  When you toss a coin, A is the event that the outcome is tails –  When you throw a dice, A is the event that the dice shows 5

§  Assume that you conduct n independent experiments •  Examples of these are

–  When you toss a coin n times in succession –  When you throw a dice n times in succession

§  Probability that the event A will occur x times •  E.g: Probability that x out of n tosses result in a tail/head

•  This is known as the Binomial Distribution

€

nx"

# $ %

& ' px 1− p( )n−x

How? Self-study

Introduction 80

Revisiting the Problem v  P (more than 10 out of n users transmits) = 0.0004 v  In our case, A is the event that a user is transmitting v  Hence, p=0.1 and x=10 v  Now, P (x> 10) = 0.0004 v  Hence, P (x<= 10) = 0.9996 v  P(x=0) + P( x=1) + P(x=2) …. = 0.9996

v  Solving this, we get n = 35 €

n0"

# $ %

& ' 0.10 1− 0.1( )n−0 +

n1"

# $ %

& ' 0.11 1− 0.1( )n−1 + ...= 0.9996

Self-study

Internet structure: network of networks

v  End systems connect to Internet via access ISPs (Internet Service Providers) §  Residential, company and university ISPs

v  Access ISPs in turn must be interconnected. v  So that any two hosts can send packets to each other

v  Resulting network of networks is very complex v  Evolution was driven by economics and national policies

v  Let’s take a stepwise approach to describe current Internet structure

1-81 Introduction

Internet structure: network of networks Question: given millions of access ISPs, how to connect them together?

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

…

… … …

…

1-82 Introduction

Internet structure: network of networks Option: connect each access ISP to every other access ISP?

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

…

… … …

…

…

…

connecting each access ISP to each other directly doesn’t

scale: O(N2) connections.

1-83 Introduction


access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

…

… … …

…

Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement.

global ISP

1-84 Introduction


access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

…

… … …

…

But if one global ISP is viable business, there will be competitors ….

ISP B

ISP A

ISP C

1-85 Introduction


access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

…

… … …

…

But if one global ISP is viable business, there will be competitors …. which must be interconnected

ISP B

ISP A

ISP C

IXP

IXP

peering link

Internet exchange point

1-86 Introduction


access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

…

… … …

…

… and regional networks may arise to connect access nets to ISPS

ISP B

ISP A

ISP C

IXP

IXP

regional net

1-87 Introduction


access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net

access net access

net

access net

…

… … …

…

… and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users

ISP B

ISP A

ISP B

IXP

IXP

regional net

Content provider network

1-88 Introduction

Introduction


v  at center: small # of well-connected large networks §  “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national &

international coverage §  content provider network (e.g, Google): private network that connects

it data centers to Internet, often bypassing tier-1, regional ISPs

1-89

access ISP

access ISP

access ISP

access ISP

access ISP

access ISP

access ISP

access ISP

Regional ISP Regional ISP

IXP IXP

Tier 1 ISP Tier 1 ISP Google

IXP

Introduction

Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

…

………

POP: point-of-presence

1-90

Introduction





1-91

Nodes and Links and Delays

A B

Introduction 1-92

Properties of Links

v  Bandwidth (capacity): “width” of the link §  number of bits sent (or received) per unit time (bits/sec or bps)

v  Propagation delay: “length” of the link §  propagation time for data to travel along the link (seconds)

v  Bandwidth-Delay Product (BDP): “volume” of the link §  amount of data that can be “in flight” at any time §  propagation delay × bits/time = total bits in link

bandwidth

Prop. Delay

delay x bandwidth

Introduction 1-93

Implications will become apparent when we study reliable data delivery and TCP

Examples of Bandwidth-Delay v  Same city over a slow link:

§  BW~100Mbps §  Propagation delay~0.1msec §  BDP ~ 10,000bits ~ 1.25KBytes

v  Cross-country over fast link:

§  BW~10Gbps §  Propagation delay~10msec §  BDP ~ 108bits ~ 12.5GBytes

v  LFN: long fat network (typically BDP > 105 bits)

NOTE: For ease of math, you should assume 1MB = 1000KB, 1KB = 1000B… Introduction 1-94

time=0

Packet Delay Sending a 100B packet from A to B?

A B

Time

1Mbps, 1ms

Time to transmit one bit = 1/106s

Time when that bit reaches B

= 1/106+1/103s

Introduction 1-95

time=0


A B

100Byte packet

Time

1Mbps, 1ms

Time to transmit 800 bits=800x1/106s

The last bit reaches B at

(800x1/106)+1/103s = 1.8ms

Introduction 1-96

time=0


A B

100Byte packet

Time

1Mbps, 1ms

Time to transmit 800 bits=800x1/106s

Time when that bit reaches B

= 1/106+1/103s


(800x1/106)+1/103s = 1.8ms

Packet Delay = Transmission Delay + Propagation Delay Packet Delay = (Packet Size ÷ Link Bandwidth) + Propagation Delay

Introduction 1-97


A B

Time

1Gbps, 1ms?


(800x1/109)+1/103s = 1.0008ms

Introduction 1-98

100Byte packet


A B

100Byte packet

Time

1Gbps, 1ms?

1GB file in 100B packets


(800x1/109)+1/103s = 1.0008ms

The last bit in the file reaches B at

(107x800x1/109)+1/103s = 8001ms

107 x 100B packets

Introduction 1-99

Introduction

Four sources of packet delay

dproc: nodal processing §  check bit errors §  determine output link §  typically < msec

A

B

propagation

transmission

nodal processing queueing

dqueue: queueing delay §  time waiting at output link

for transmission §  depends on congestion

level of router

dnodal = dproc + dqueue + dtrans + dprop

1-100

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 dprop

very different

Four sources of packet delay

propagation

nodal processing queueing

dnodal = dproc + dqueue + dtrans + dprop

1-101

A

B

transmission

Interactive Java Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/transmission/delay.html

Introduction

Caravan analogy

v  cars “propagate” at 100 km/hr

v  toll booth takes 12 sec to service car (bit transmission time)

v  car~bit; caravan ~ packet v  Q: How long until caravan is

lined up before 2nd toll booth?

§  time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec

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

§  A: 62 minutes

toll booth

toll booth

ten-car caravan

100 km 100 km

1-102

Self-study

Introduction

Caravan analogy (more)

v  suppose cars now “propagate” at 1000 km/hr v  and suppose toll booth now takes one min to service a car v  Q: Will cars arrive to 2nd booth before all cars serviced at first

booth?

§  A: Yes! after 7 min, 1st car arrives at second booth; three cars still at 1st booth.

toll booth

toll booth

ten-car caravan

100 km 100 km

1-103

Self-study

Host A Host B transmission 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?

Answer: First bit arrives after 1/R + d/s = 1/106 + 103/(2x108) = 10-6 + 5x10-6 = 6x10-6 = 6 µsec After 6 µsec 6 bits are already transmitted; so 7th bit is being transmitted

1-104 Introduction

Quiz: Delays

Transmission vs. propagation

Host A Host B transmission rate R = ??

distance = 2 km, speed = 2x108m/s

L=100Bytes

Question: At what rate (bandwidth) R would the propagation delay equal the transmission delay of packet?

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

1-105 Introduction

Quiz: Delays

Queueing delay (more insight)

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

aL/R is called traffic intensity

queue Packet arrival rate = a packets/sec

Link bandwidth = R bits/sec


Introduction 1-106

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)

queue Link bandwidth = R bits/sec 1 packet arrives

every L/R seconds

Packet length L bits

Introduction 1-107

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 is 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

queue Link bandwidth = R bits/sec N packet arrive simultaneously

every LN/R seconds

Packet length L bits

Introduction 1-108

Queueing delay: behaviour

q  La/R ~ 0: avg. queueing delay small q  La/R -> 1: delays become large q  La/R > 1: more “work” than can be

serviced, average delay infinite! (this is when a is random!)

queue Packet arrival rate = a packets/sec

Link bandwidth = R bits/sec


Interactive Java Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/queuing/queuing.html

Introduction 1-109

Introduction

“Real” Internet delays and routes v  what do “real” Internet delay & loss look like? v  traceroute program: provides delay

measurement from source to router along end-end Internet path towards destination. For all i: §  sends three packets that will reach router i on path

towards destination §  router i will return packets to sender §  sender times interval between transmission and reply.

3 probes

3 probes

3 probes

1-110

Your first lab

Introduction

“Real” Internet delays, routes

1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms

traceroute: gaia.cs.umass.edu to www.eurecom.fr 3 delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu

* means no response (probe lost, router not replying)

trans-oceanic link

1-111 * Do some traceroutes from exotic countries at www.traceroute.org

Your first lab

v  Propagation delay depends on the size of the packet. True or false?

A.  True

B.  False

Introduction 1-112

Quiz: Delays

v  Which of the following delays is significantly affected by the load in the network?

A.  Processing delay

B.  Queuing delay

C.  Transmission delay

D.  Propagation delay

Introduction 1-113

Quiz: Delays

v  Consider a packet that has just arrived at a router. What is the correct order of the delays encountered by the packet until it reaches the next-hop router?

A.  Transmission, processing, propagation, queuing

B.  Propagation, processing, transmission, queuing

C.  Processing, queuing, transmission, propagation

D.  Queuing, processing, propagation, transmission

Introduction 1-114

Quiz: Delays

http://gaia.cs.umass.edu/kurose_ross/interactive/one-hop-delay.php

http://gaia.cs.umass.edu/kurose_ross/interactive/end-end-delay.php

Introduction 1-115

Quiz: Solve on your own

Packet switching: store and forward

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

R L d

q  distance = d meters; speed of propagation = s m/sec q  transmission rate of link = R bits/s

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

Example: q  d/s = 0.5 sec q  L = 10 Mbits q  R = 1 Mbps q  delay = 10.5 sec

Example: q  d/s = 0.5 sec q  L = 10 Mbits q  R = 1 Mbps q  delay = 20.5 sec

R R L

d/2 d/2

1-116 Introduction

Store-and-forward & queuing delay

q  Case 1: Assume R1 < R2

q  distance = d meters; speed of propagation = s m/sec q  transmission rate of link = R1 and R2 bits/s q  Consider sending two packets A and B back to back

R1 R2 L

d

q  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/R2 Queueing delay = d2 – d1 = L/R2 – L/R1 if positive, otherwise 0. Hence: when R1 < R2, queueing delay = d2 – d1 = 0 when R1 > R2, queueing delay = d2 – d1 = L/R2 – L/R1

1-117 Introduction

d/2 d/2

Introduction

Packet loss v  queue (aka buffer) preceding link in buffer has finite

capacity v  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 to full buffer is lost

buffer (waiting area)

1-118

Interactive Java Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/queuing/queuing.html

Introduction

Throughput

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, with file of F bits

to send to client

link capacity Rs bits/sec

link capacity Rc bits/sec

server sends bits (fluid) into pipe

pipe that can carry fluid at rate Rs bits/sec)

pipe that can carry fluid at rate Rc bits/sec)

1-119

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 throughput bottleneck link

Rs bits/sec Rc bits/sec

1-120

Introduction

Throughput: Internet scenario

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

Rs

Rs

Rs

Rc

Rc

Rc

R

v  per-connection end-end throughput: min(Rc,Rs,R/10)

v  in practice: Rc or Rs is often bottleneck

1-121

Introduction

Introduction: summary

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

network §  packet-switching versus

circuit-switching §  Internet structure

v  performance: loss, delay, throughput

v  Next Week §  Protocol layers, service models §  Network security overview

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

of networking v  more depth, detail to

follow!

1-122

Solve all remaining problems Reading Exercise Chapter 1: 1.5 – 1.8 Chapter 2: 2.1 – 2.4

Documents

Week 1 - WebCMS3