CS计算机代考程序代写 Java 1.Intro_Networks

1.Intro_Networks

Introduction to Computer Networks

Computer Networks and Applications

Week 1

COMP 3331/COMP 9331

Reading Guide: Chapter 1, Sections 1.1 – 1.4

Acknowledgment

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

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. 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

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

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 (answer in comments on Zeeting)

Open a browser and type: www.zeetings.com/salil

Answers: B and C are valid answers

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

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

vcommunication links
§ fiber, copper, radio,

satellite
§ transmission rate:

bandwidth

vPacket switches: forward
packets (chunks of data)
§ routers and link layer

switches

wired
links

wireless
links

router

mobile network

global ISP

regional ISP

home
network

institutional
network

smartphone

server

wireless
laptop

“Fun” Internet appliances

Picture frame

Web-enabled toaster +
weather forecaster

Smart Lightbulbs
Internet
refrigerator

Networked TV Set top Boxes

Tweet-a-watt:
monitor energy use

sensorized,
bed
mattress

car

pacemaker

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

institutional
network

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

institutional
network

Computer)Networks,)Fall)2015 8

while'(…)'{‘
”’message’=’…;’
”’send'(‘message,’…’);’
}

while'(…)'{‘
”’message’=’receive'(‘…’);’
}

Alice

Bob

Computer)Networks,)Fall)2015

while'(…)'{‘
”’message’=’receive'(‘…’);’
}

ApplicaGon-
Programming-
InterfaceAlice

while'(…)'{‘
”’message’=’…;’
”’send'(‘message,’…’);’
}

Bob

13
Computer)Networks,)Fall)2015 7

instant-messaging

facebook-
server

firefox-accessing-
facebook

world-of-warcraE-
client

world-of-warcraE-
server

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

a human protocol and a computer network protocol:

Q: other human protocols?

Got the
time?
2:00

TCP connection
response

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

time

TCP connection
request

What’s a protocol?

Quiz: Internet of Things
How many Internet-connected devices do you have in your
home (include your computers, phones, tablets)?

A. Less than 10

B. Between 10 to 20

C. Between 20 to 50

D. Between 50 to 100

E. More than 100

Open a browser and type: www.zeetings.com/salil

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, service models
1.6 networks under attack: security
1.7 history

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

institutional
network

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?

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

DSL
modem

splitter

DSL access
multiplexer

Access net: digital subscriber line (DSL)

ADSL over POTS
voice, data transmitted

at different frequencies over
dedicated line to central office

Ø Different data rates for upload and download (ADSL)
• < 2.5 Mbps upstream transmission rate (typically < 1 Mbps) • < 24 Mbps downstream transmission rate (typically < 10 Mbps) DSL modem splitter 21 Low -pa ss f ilter for voic e High-pass filter for data Access net: digital subscriber line (DSL) 22 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 23 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 24 Fiber to the home/premise/curb v Fully optical fiber path all the way to the home (or premise or curb) § e.g., NBN, Google, Verizon FIOS § ~30 Mbps to 1Gbps 25 Access net: home network to/from headend or central office cable or DSL or Fiber modem router, firewall, NAT wired Ethernet (1 Gbps) wireless access point (54 Mbps) wireless devices often combined in single box 26 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) 27 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/n (WiFi): 11, 54, 300 Mbps transmission rate § 802.11ac: 1 Gpbs(2.4GHz) + 4.34Gbps (5GHz) § 802.11ax: WiFi 6 wide-area wireless access § provided by telco (cellular) operator, 10’s km § between 10 and 100 Mbps § 4G, 5G to Internet to Internet 28 29 Sample results FTTC + Cable + WiFi @ my home Wired Network @ CSEUniwide 4G Network Can you explain the differences? 30 Quiz: Your access network Your residential ISP provides connectivity using the following technology: A. DSL B. Cable C. Fiber to the home/premise/curb D. Mobile (3G/4G/5G) E. Satellite F. Something Else (type in Zeetings comment) Open a browser and type: www.zeetings.com/salil 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 31 Self Study 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 32 twisted pair (TP) v two insulated copper wires § Category 5: 100 Mbps, 1 Gpbs Ethernet § Category 6: 10Gbps Self Study 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, 450 Mbps, Gbps v wide-area (e.g., cellular) § 4G cellular: ~ 10 Mbps v satellite § Kbps to 45Mbps channel (or multiple smaller channels) § 270 msec end-end delay § geosynchronous versus low earth-orbiting (LEO) 33 Self Study 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, service models 1.6 networks under attack: security 1.7 history 34 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 35 Circuit Switching end-end resources allocated to, reserved for “call” between source & dest: § in diagram, each link has four circuits. • call gets 2nd circuit in top link and 1st circuit in right link. § dedicated resources: no sharing • circuit-like (guaranteed) performance § circuit segment idle if not used by call (no sharing) § commonly used in traditional telephone networks 36 Circuit switching: FDM versus TDM FDM frequency timeTDM frequency time 4 users Example: 37 Information time Timing in Circuit Switching Circuit Establish ment Transfer Circuit Tear- down 38 Why circuit switching is not feasible? Ø Inefficient • Computer communications tends to be very bursty. For example, viewing a sequence of web pages • Dedicated circuit cannot be used or shared in periods of silence • Cannot adopt to network dynamics 39 Ø Fixed data rate • Computers communicate at very diverse rates. For example, viewing a video vs using telnet or web browsing • Fixed data rate is not useful Ø Connection state maintenance • Requires per communication state to be maintained that is a considerable overhead • Not scalable 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 HeaderData header payload 40 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) 41 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 42 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 43 time Timing in Packet Switching paylo ad h d r What about the time to process the packet at the switch? • We’ll assume it’s relatively negligible (mostly true) 44 time Timing in Packet Switching paylo ad h d r Could the switch start transmitting as soon as it has processed the header? 45 time Timing in Packet Switching paylo ad h d r Could the switch start transmit as soon as it has processed the header? Yes! This would be called a “cut through” switch 46 time Timing in Packet Switching paylo ad h d r We will always assume a switch processes/forwards a packet after it has received it entirely. This is called “store and forward” switching 47 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 48 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” 49 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 50 Data Rate 1 Data Rate 2 Data Rate 3 Three Flows with Bursty Traffic Time Time Time Capacity 51 Data Rate 1 Data Rate 2 Data Rate 3 When Each Flow Gets 1/3rd of Capacity Time Time Time Overloaded 52 like circuit switching 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 53 packet switching Data Rate 1 Data Rate 2 Data Rate 3 Three Flows with Bursty Traffic Time Time Time Capacity 54 Data Rate 1 Data Rate 2 Data Rate 3 Three Flows with Bursty Traffic Time Time Time Capacity 55 Data Rate 1+2+3 >> Capacity

Three Flows with Bursty Traffic

Time

Time
Capacity

What do we do under overload?
56

time à

B
W

pkt tx
time

Statistical multiplexing: pipe view

No Overload

Statistical multiplexing: pipe view

Transient Overload
Not such a rare event

Queue overload
into Buffer

Statistical multiplexing: pipe view

Transient Overload
Not such a rare event

Queue overload
into Buffer

Statistical multiplexing: pipe view

Transient Overload
Not such a rare event

Queue overload
into Buffer

Transient Overload
Not such a rare event

Queue overload
into Buffer

Statistical multiplexing: pipe view

Transient Overload
Not such a rare event

Queue overload
into Buffer

Statistical multiplexing: pipe view

Transient Overload
Not a rare event!Buffer absorbs transient bursts

Queue overload
into Buffer

Statistical multiplexing: pipe view

What about persistent overload?
Will eventually drop packets

Queue overload
into Buffer

Statistical multiplexing: pipe view

Packet switching versus circuit switching

example:
§ 1 Mb/s link
§ each user:

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

vcircuit-switching:
§ 10 users

vpacket 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
say 70?

…
..

Hint: Bernoulli Trials and Binomial Distribution

Binomial Probability Distribution

v A fixed number of observations (trials), n
§ E.g., 5 tosses of a coin

v Binary random variable
§ E.g., head or tail in a coin toss
§ Often called as success or failure
§ Probability of success is p and failure is (1-p)

v Constant probability for each observation

Binomial Distribution: Example

v Q: What is the probability of observing exactly 3
heads in a sequence of 5 coin tosses

v A:
§ One way to get exactly 3 heads is: HHHTT
§ Probability of this sequence occurring = (1/2) x (1/2) x

(1/2) x (1-1/2) x (1-1/2) = (1/2)5

§ Another way to get exactly 3 heads is: THHHT
§ Probability of this sequence occurring = (1-1/2) x (1/2)

x (1/2) x (1/2) x (1-1/2) = (1/2)5

§ How many such unique combinations exist?

Binomial Distribution: Example

70P (3 heads and 2 tails) = 10 x (1/2)5 = 0.3125

Binomial Distribution

Packet switching versus circuit switching

v Let’s revisit the earlier problem
v N = 35 users
v Prob (# active users > 10)= 1– Prob (# active = 10)

– Prob (# active = 9)
– Prob (# active = 8)
…
– Prob (# active = 0)

where Prob (# active = 10) = C(35,10) x 0.110 x 0.925

v Prob (# active users > 10) = 0.0004 (approx)
72

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

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

Quiz: Switching

In ____________ resources are allocated on
demand

A. Packet switching

B. Circuit switching

C. Both

D. None

Open a browser and type: www.zeetings.com/salil

Answer: A

Quiz: Switching

A message from device A to B consists of packet X
and packet Y. In a circuit switched network, packet
Y’s path ___________________ packet X’s path

A. is the same

B. is independent

C. is always different from

Open a browser and type: www.zeetings.com/salil

Answer: A

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

Internet structure: network of networks

Question: given millions of access ISPs, how to connect them
together?

access
net

access
netaccess

net

access
net

…
…

……
…

…

Internet structure: network of networks

Option: connect each access ISP to every other access ISP?

access
net

access
netaccess

net

access
net

…
…

……
…

…

…
……

connecting each access ISP
to each other directly doesn’t

scale: O(N2) connections.

Internet structure: network of networks

access
net

access
netaccess

net

access
net

…
…

……
…

…

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

global
ISP

Internet structure: network of networks

access
net

access
netaccess

net

access
net

…
…

……
…

…

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

ISP B

ISP A

ISP C

Internet structure: network of networks

access
net

access
netaccess

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

peering link

Internet exchange point

Internet structure: network of networks

access
net

access
netaccess

net

access
net

…
…

……
…

…

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

ISP B

ISP A

ISP C

IXP

regional net

Internet structure: network of networks

access
net

access
netaccess

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

regional net

Content provider network

Internet structure: network of networks

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

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

it data centers to Internet, often bypassing tier-1, regional ISPs 84

access
ISP

Regional ISP Regional ISP

IXP IXP

Tier 1 ISP Tier 1 ISP Google

IXP

1-85

AARNET: Australia’s Academic and
Research Network
v https://www.aarnet.edu.au/
v https://www.submarinecablemap.com

https://www.aarnet.edu.au/
https://www.submarinecablemap.com/

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, service models
1.6 networks under attack: security
1.7 history

How do loss and delay occur?
Packets queue in router buffers
Ø Packet arrival rate to link (temporarily) exceeds output link

capacity
Ø Packets queue, wait for turn

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets
dropped (loss) if no free buffers

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 88 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/sdtrans and dprop very different Four sources of packet delay propagation nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop 89 A B transmission Caravan analogy Ø Car ~bit; Caravan ~ packet Ø Cars “propagate” at 100 km/hr Ø Toll booth takes 12 sec to service car (bit transmission time) Ø 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 90 Caravan analogy (more) Ø Suppose cars now “propagate” at 1000 km/hr Ø And suppose toll booth now takes one min to service a car Ø 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 91 Interactive Java Applet – Propagation vs transmission delay https://www2.tkn.tu-berlin.de/teaching/rn/animations/propagation/ 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

Packet length
= L bits

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

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 at 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

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

Packet length
= L bits

Interactive Java Applet:
http://computerscience.unicam.it/marcantoni/reti/applet/QueuingAndLossInteractive/1.html

End to End Delay

d1,r1 d2,r2 d3,r3

R1 R2Client Server

Client

Server

Time
R1

Packet length = L
Propagation speed = s

d1/s

d2/s

d3/s

L/r1

L/r2

L/r3
Queueing Delay

d3/s

L/r3

In the picture, r1 = r2 = r3, you may wish to consider what happens when this is not the case

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

“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

98
* Do some traceroutes from countries at www.traceroute.org

“Real” delay variations
dnodal = dproc + dqueue + dtrans + dprop

End-to-end delay = sum of all dnodal along the path

100

Quiz: Propagation Delay

Propagation delay depends on the size of the packet

A. True

B. False

Open a browser and type: www.zeetings.com/salil

Answer: B

101

Quiz: Oh these delays

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

Open a browser and type: www.zeetings.com/salil

Answer: C

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

packet being transmitted

packet arriving to
full buffer is lost

buffer
(waiting area)

102

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)

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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

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Throughput: Internet scenario

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

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

v in practice: Rc or Rs
is often bottleneck

105

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
§ Application Layer

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