1 of 88

Introduction: 1-1

Chapter 1�Introduction

A note on the use of these PowerPoint slides:

We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:

​

​

  • If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!)
  • If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

​

For a revision history, see the slide note for this page.

​

Thanks and enjoy! JFK/KWR

​

All material copyright 1996-2023

J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking: A Top-Down Approach �8th edition �Jim Kurose, Keith Ross�Pearson, 2020

2 of 88

Chapter 1: introduction

Chapter goal:

  • Get “feel,” “big picture,” introduction to terminology
    • more depth, detail later in course

Overview/roadmap:

  • What is the Internet? What is a protocol?
  • Network edge: hosts, access network, physical media
  • Network core: packet/circuit switching, internet structure
  • Performance: loss, delay, throughput
  • Protocol layers, service models
  • Security
  • History

​

Introduction: 1-2

3 of 88

The Internet: a “nuts and bolts” view

Introduction: 1-3

Internet

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

Packet switches: forward packets (chunks of data)

    • routers, switches

​

Communication links

    • fiber, copper, radio, satellite
    • transmission rate: bandwidth�

Billions of connected computing devices:

    • hosts = end systems
    • running network apps at Internet’s “edge”

Networks

    • collection of devices, routers, links: managed by an organization

4 of 88

“Fun” Internet-connected devices

Introduction: 1-4

Web-enabled toaster +

weather forecaster

Internet phones

Slingbox: remote

control cable TV

Security Camera

IP picture frame

Internet

refrigerator

Tweet-a-watt:

monitor energy use

sensorized,

bed

mattress

Amazon Echo

Others?

Gaming devices

cars

scooters

bikes

Pacemaker & Monitor

AR devices

Fitbit

diapers

5 of 88

The Internet: a “nuts and bolts” view

  • Internet: “network of networks”
    • Interconnected ISPs

​

Introduction: 1-5

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

  • protocols are everywhere
    • control sending, receiving of messages
    • e.g., HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4/5G, Ethernet

​

Ethernet

HTTP

Skype

IP

WiFi

4G

TCP

Streaming

video

  • Internet standards
    • RFC: Request for Comments
    • IETF: Internet Engineering Task Force

​

6 of 88

The Internet: a “services” view

  • Infrastructure that provides services to applications:
    • Web, streaming video, multimedia teleconferencing, email, games, e-commerce, social media, inter-connected appliances, …

​

​

Introduction: 1-6

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

HTTP

Skype

Streaming

video

  • provides programming interface to distributed applications:
    • “hooks” allowing sending/receiving apps to “connect” to, use Internet transport service
    • provides service options, analogous to postal service

​

​

7 of 88

What’s a protocol?

Introduction: 1-7

Human protocols:

  • “what’s the time?”
  • “I have a question”
  • introductions

Network protocols:

  • computers (devices) rather than humans
  • all communication activity in Internet governed by protocols

Protocols define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt

Rules for:

… specific messages sent

… specific actions taken when message received, or other events

8 of 88

What’s a protocol?

Introduction: 1-8

A human protocol and a computer network protocol:

​

Q: other human protocols?

Hi

Hi

Got the

time?

2:00

time

TCP connection

response

<file>

TCP connection

request

GET http://gaia.cs.umass.edu/kurose_ross

9 of 88

Chapter 1: roadmap

  • What is the Internet?
  • What is a protocol?
  • Network edge: hosts, access network, physical media
  • Network core: packet/circuit switching, internet structure
  • Performance: loss, delay, throughput
  • Security
  • Protocol layers, service models
  • History

​

Introduction: 1-9

10 of 88

A closer look at Internet structure

Network edge:

  • hosts: clients and servers
  • servers often in data centers

​

​

Introduction: 1-10

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

11 of 88

A closer look at Internet structure

Network edge:

  • hosts: clients and servers
  • servers often in data centers

Access networks, physical media:

  • wired, wireless communication links

​

​

Introduction: 1-11

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

12 of 88

A closer look at Internet structure

Network edge:

  • hosts: clients and servers
  • servers often in data centers

Access networks, physical media:

  • wired, wireless communication links

Network core:

    • interconnected routers
    • network of networks

​

​

Introduction: 1-12

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

13 of 88

Access networks and physical media

Q: How to connect end systems to edge router?

  • residential access nets
  • institutional access networks (school, company)
  • mobile access networks (WiFi, 4G/5G)

​

​

Introduction: 1-13

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

14 of 88

Access networks: cable-based access

Introduction: 1-14

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 (FDM): different channels transmitted in different frequency bands

15 of 88

Access networks: cable-based access

Introduction: 1-15

cable

modem

splitter

…

cable headend

data, TV transmitted at different

frequencies over shared cable

distribution network

  • HFC: hybrid fiber coax
    • asymmetric: up to 40 Mbps – 1.2 Gbps downstream transmission rate, 30-100 Mbps upstream transmission rate
  • network of cable, fiber attaches homes to ISP router
    • homes share access network to cable headend

​

cable modem

termination system

CMTS

ISP

16 of 88

Access networks: digital subscriber line (DSL)

Introduction: 1-16

ISP

central office

telephone

network

DSLAM

voice, data transmitted

at different frequencies over

dedicated line to central office

​

    • 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
    • 24-52 Mbps dedicated downstream transmission rate
    • 3.5-16 Mbps dedicated upstream transmission rate

DSL

modem

splitter

DSL access

multiplexer

17 of 88

Access networks: home networks

Introduction: 1-17

to/from headend or central office

cable or DSL modem

router, firewall, NAT

wired Ethernet (1 Gbps)

WiFi wireless access

point (54, 450 Mbps)

Wireless and wired

devices

often combined

in single box

18 of 88

Wireless access networks

Introduction: 1-18

Shared wireless access network connects end system to router

  • via base station aka “access point”

Wireless local area networks (WLANs)

    • typically within or around building (~100 ft)
    • 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate

to Internet

to Internet

​

Wide-area cellular access networks

    • provided by mobile, cellular network operator (10’s km)
    • 10’s Mbps
    • 4G/5G cellular networks

19 of 88

Access networks: enterprise networks

Introduction: 1-19

  • companies, universities, etc.
    • mix of wired, wireless link technologies, connecting a mix of switches and routers (we’ll cover differences shortly)
      • Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps
      • WiFi: wireless access points at 11, 54, 450 Mbps

Ethernet

switch

institutional mail,

web servers

institutional router

Enterprise link to

ISP (Internet)

​

20 of 88

Access networks: data center networks

Introduction: 1-20

  • high-bandwidth links (10s to 100s Gbps) connect hundreds to thousands of servers together, and to Internet

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

Courtesy: Massachusetts Green High Performance Computing Center (mghpcc.org)

21 of 88

Host: sends packets of data

Introduction: 1-21

host sending function:

  • takes application message
  • breaks into smaller chunks, known as packets, of length L bits
  • 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)

=

=

22 of 88

Links: physical media

Introduction: 1-22

  • bit: propagates between�transmitter/receiver pairs
  • physical link: what lies between transmitter & receiver
  • guided media:
    • signals propagate in solid media: copper, fiber, coax
  • unguided media:
    • signals propagate freely, e.g., radio

Twisted pair (TP)

  • two insulated copper wires
    • Category 5: 100 Mbps, 1 Gbps Ethernet
    • Category 6: 10Gbps Ethernet

23 of 88

Links: physical media

Introduction: 1-23

Coaxial cable:

  • two concentric copper conductors
  • bidirectional
  • broadband:
    • multiple frequency channels on cable
    • 100’s Mbps per channel

Fiber optic cable:

  • glass fiber carrying light pulses, each pulse a bit
  • high-speed operation:
    • high-speed point-to-point transmission (10’s-100’s Gbps)
  • low error rate:
    • repeaters spaced far apart
    • immune to electromagnetic noise

24 of 88

Links: physical media

Introduction: 1-24

Wireless radio

  • signal carried in various “bands” in electromagnetic spectrum
  • no physical “wire”
  • broadcast, “half-duplex” (sender to receiver)
  • propagation environment effects:
    • reflection
    • obstruction by objects
    • Interference/noise

Radio link types:

  • Wireless LAN (WiFi)
    • 10-100’s Mbps; 10’s of meters
  • wide-area (e.g., 4G/5G cellular)
    • 10’s Mbps (4G) over ~10 Km
  • Bluetooth: cable replacement
    • short distances, limited rates
  • terrestrial microwave
    • point-to-point; 45 Mbps channels
  • satellite
    • up to < 100 Mbps (Starlink) downlink
    • 270 msec end-end delay (geostationary)

25 of 88

Chapter 1: roadmap

  • What is the Internet?
  • What is a protocol?
  • Network edge: hosts, access network, physical media
  • Network core: packet/circuit switching, internet structure
  • Performance: loss, delay, throughput
  • Security
  • Protocol layers, service models
  • History

​

Introduction: 1-25

26 of 88

The network core

  • mesh of interconnected routers
  • packet-switching: hosts break application-layer messages into packets
    • network forwards packets from one router to the next, across links on path from source to destination

​

​

Introduction: 1-26

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

27 of 88

Two key network-core functions

Forwarding:

  • aka “switching”
  • local action: move arriving packets from router’s input link to appropriate router output link

​

Introduction: 1-27

1

2

3

0111

destination address in arriving

packet’s header

routing algorithm

header value

output link

0100

0101

0111

1001

3

2

2

1

local forwarding table

local forwarding table

Routing:

  • global action: determine source-destination paths taken by packets
  • routing algorithms

​

routing algorithm

28 of 88

Introduction: 1-28

routing

29 of 88

Introduction: 1-29

forwarding

forwarding

30 of 88

Packet-switching: store-and-forward

  • packet transmission delay: takes L/R seconds to transmit (push out) L-bit packet into link at R bps
  • store and forward: entire packet must arrive at router before it can be transmitted on next link

​

​

​

Introduction: 1-30

source

R bps

destination

1

2

3

L bits

per packet

R bps

One-hop numerical example:

  • L = 10 Kbits
  • R = 100 Mbps
  • one-hop transmission delay = 0.1 msec

31 of 88

Packet-switching: queueing

Introduction: 1-31

A

B

C

R = 100 Mb/s

R = 1.5 Mb/s

D

E

queue of packets

waiting for transmission over output link

Queueing occurs when work arrives faster than it can be serviced:

32 of 88

Packet-switching: queueing

Packet queuing and loss: if arrival rate (in bps) to link exceeds transmission rate (bps) of link for some period of time:

  • packets will queue, waiting to be transmitted on output link
  • packets can be dropped (lost) if memory (buffer) in router fills up

​

Introduction: 1-32

A

B

C

R = 100 Mb/s

R = 1.5 Mb/s

D

E

queue of packets

waiting for transmission over output link

33 of 88

Alternative to packet switching: circuit switching

end-end resources allocated to, reserved for “call” between source and destination

  • 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)

​

​

Introduction: 1-33

* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive

  • commonly used in traditional telephone networks

​

​

34 of 88

Circuit switching: FDM and TDM

Frequency Division Multiplexing (FDM)

  • optical, electromagnetic frequencies divided into (narrow) frequency bands

​

Introduction: 1-34

frequency

time

frequency

time

4 users

Time Division Multiplexing (TDM)

  • time divided into slots
  • each call allocated its own band, can transmit at max rate of that narrow band

​

  • each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band (only) during its time slot(s)

35 of 88

Packet switching versus circuit switching

Introduction: 1-35

example:

  • 1 Gb/s link
  • each user:
    • 100 Mb/s when “active”
    • active 10% of time

​

​

Q: how many users can use this network under circuit-switching and packet switching?

* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive

N

users

1 Gbps link

…..

​

  • circuit-switching: 10 users

​

Q: how did we get value 0.0004?

A: HW problem (for those with course in probability only)

​

  • packet switching: with 35 users, probability > 10 active at same time is less than .0004 *

​

36 of 88

Packet switching versus circuit switching

Introduction: 1-36

  • great for “bursty” data – sometimes has data to send, but at other times not
    • resource sharing
    • simpler, no call setup
  • excessive congestion possible: packet delay and loss due to buffer overflow
    • protocols needed for reliable data transfer, congestion control
  • Q: How to provide circuit-like behavior with packet-switching?
    • “It’s complicated.” We’ll study various techniques that try to make packet switching as “circuit-like” as possible.

Is packet switching a “slam dunk winner”?

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

37 of 88

Internet structure: a “network of networks”

  • hosts connect to Internet via access Internet Service Providers (ISPs)
  • access ISPs in turn must be interconnected
      • so that any two hosts (anywhere!) can send packets to each other
  • resulting network of networks is very complex
      • evolution driven by economics, national policies

​

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

​

mobile network

home network

enterprise

network

national or global ISP

local or regional ISP

datacenter

network

content

provider

network

38 of 88

Internet structure: a “network of networks”

Introduction: 1-38

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

…

…

…

…

…

…

39 of 88

Internet structure: a “network of networks”

Introduction: 1-39

…

…

…

…

…

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

…

…

…

…

…

…

connecting each access ISP to each other directly doesn’t scale: O(N2) connections.

40 of 88

Internet structure: a “network of networks”

Introduction: 1-40

Option: connect each access ISP to one global transit ISP?

Customer and provider ISPs have economic agreement.

global�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

…

…

…

…

…

…

41 of 88

Internet structure: a “network of networks”

Introduction: 1-41

ISP A

ISP C

ISP B

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 ….

42 of 88

Internet structure: a “network of networks”

Introduction: 1-42

ISP A

ISP C

ISP B

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 …. who will want to be connected

IXP

peering link

Internet exchange point

IXP

43 of 88

Internet structure: a “network of networks”

Introduction: 1-43

ISP A

ISP C

ISP B

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

IXP

IXP

access

net

access

net

regional ISP

access

net

access

net

access

net

44 of 88

Internet structure: a “network of networks”

Introduction: 1-44

ISP A

ISP C

ISP B

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

IXP

IXP

access

net

access

net

access

net

access

net

access

net

Content provider network

regional ISP

45 of 88

Internet structure: a “network of networks”

Introduction: 1-45

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

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 networks (e.g., Google, Facebook): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs

​

Regional ISP

Regional ISP

Tier 1 ISP

Tier 1 ISP

IXP

Google

IXP

IXP

46 of 88

Chapter 1: roadmap

  • What is the Internet?
  • What is a protocol?
  • Network edge: hosts, access network, physical media
  • Network core: packet/circuit switching, internet structure
  • Performance: loss, delay, throughput
  • Security
  • Protocol layers, service models
  • History

​

Introduction: 1-46

47 of 88

How do packet delay and loss occur?

  • packets queue in router buffers, waiting for turn for transmission
    • queue length grows when arrival rate to link (temporarily) exceeds output link capacity
  • packet loss occurs when memory to hold queued packets fills up

Introduction: 1-47

A

B

packet being transmitted (transmission delay)

packets in buffers (queueing delay)

free (available) buffers: arriving packets

dropped (loss) if no free buffers

48 of 88

Packet delay: four sources

Introduction: 1-48

dproc: nodal processing

  • check bit errors
  • determine output link
  • typically < microsecs

dqueue: queueing delay

  • time waiting at output link for transmission
  • depends on congestion level of router

propagation

nodal

processing

queueing

dnodal = dproc + dqueue + dtrans + dprop

A

B

transmission

49 of 88

Packet delay: four sources

Introduction: 1-49

propagation

nodal

processing

queueing

dnodal = dproc + dqueue + dtrans + dprop

A

B

transmission

dtrans: transmission delay:

  • L: packet length (bits)
  • R: link transmission rate (bps)
  • dtrans = L/R

dprop: propagation delay:

  • d: length of physical link
  • s: propagation speed (~2x108 m/sec)
  • dprop = d/s

dtrans and dprop

very different

50 of 88

Caravan analogy

Introduction: 1-50

  • car ~ bit; caravan ~ packet; toll service ~ link transmission
  • toll booth takes 12 sec to service car (bit transmission time)
  • “propagate” at 100 km/hr
  • 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

(aka link)

ten-car caravan

(aka 10-bit packet)

100 km

100 km

toll booth

toll booth

(aka link)

toll booth

51 of 88

Caravan analogy

Introduction: 1-51

toll booth

toll booth

(aka router)

ten-car caravan

(aka 10-bit packet)

100 km

100 km

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

​

A: Yes! after 7 min, first car arrives at second booth; three cars still at first booth

52 of 88

Packet queueing delay (revisited)

Introduction: 1-52

  • a: average packet arrival rate
  • L: packet length (bits)
  • R: link bandwidth (bit transmission rate)
  • La/R ~ 0: avg. queueing delay small
  • La/R -> 1: avg. queueing delay large
  • La/R > 1: more “work” arriving is more than can be serviced - average delay infinite!

La/R ~ 0

La/R -> 1

traffic intensity = La/R

average queueing delay

1

service rate of bits

R

arrival rate of bits

L

a

.

:

“traffic

intensity”

53 of 88

“Real” Internet delays and routes

Introduction: 1-53

  • what do “real” Internet delay & loss look like?
  • traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i:

3 probes

3 probes

3 probes

    • sends three packets that will reach router i on path towards destination (with time-to-live field value of i)
    • router i will return packets to sender
    • sender measures time interval between transmission and reply

54 of 88

Real Internet delays and routes

Introduction: 1-54

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

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

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

3 delay measurements from

gaia.cs.umass.edu to cs-gw.cs.umass.edu

3 delay measurements

to border1-rt-fa5-1-0.gw.umass.edu

looks like delays decrease! Why?

trans-oceanic link

55 of 88

Packet loss

Introduction: 1-55

  • queue (aka buffer) preceding link in buffer has finite capacity

A

B

packet being transmitted

buffer

(waiting area)

* Check out the Java applet for an interactive animation (on publisher’s website) of queuing and loss

packet arriving to

full buffer is lost

  • packet arriving to full queue dropped (aka lost)
  • lost packet may be retransmitted by previous node, by source end system, or not at all

56 of 88

Throughput

Introduction: 1-56

  • throughput: rate (bits/time unit) at which bits are being sent from sender to 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)

57 of 88

Throughput

Introduction: 1-57

Rs < Rc What is average end-end throughput?

Rs bits/sec

Rc bits/sec

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

58 of 88

Throughput: network scenario

Introduction: 1-58

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

Rs

Rs

Rs

Rc

Rc

Rc

R

  • per-connection end-end throughput: min(Rc,Rs,R/10)
  • in practice: Rc or Rs is often bottleneck

* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/

59 of 88

Chapter 1: roadmap

  • What is the Internet?
  • What is a protocol?
  • Network edge: hosts, access network, physical media
  • Network core: packet/circuit switching, internet structure
  • Performance: loss, delay, throughput
  • Security
  • Protocol layers, service models
  • History

​

Introduction: 1-59

60 of 88

Network security

Introduction: 1-60

  • Internet not originally designed with (much) security in mind
    • original vision: “a group of mutually trusting users attached to a transparent network” ☺
    • Internet protocol designers playing “catch-up”
    • security considerations in all layers!
  • We now need to think about:
    • how bad guys can attack computer networks
    • how we can defend networks against attacks
    • how to design architectures that are immune to attacks

61 of 88

Network security

Introduction: 1-61

  • Internet not originally designed with (much) security in mind
    • original vision: “a group of mutually trusting users attached to a transparent network” ☺
    • Internet protocol designers playing “catch-up”
    • security considerations in all layers!
  • We now need to think about:
    • how bad guys can attack computer networks
    • how we can defend networks against attacks
    • how to design architectures that are immune to attacks

62 of 88

Bad guys: packet interception

Introduction: 1-62

packet “sniffing”:

    • broadcast media (shared Ethernet, wireless)
    • promiscuous network interface reads/records all packets (e.g., including passwords!) passing by

A

B

C

src:B dest:A payload

Wireshark software used for our end-of-chapter labs is a (free) packet-sniffer

​

63 of 88

Bad guys: fake identity

Introduction: 1-63

IP spoofing: injection of packet with false source address

A

B

C

src:B dest:A payload

64 of 88

Bad guys: denial of service

Introduction: 1-64

target

Denial of Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic

1. select target

2. break into hosts around the network (see botnet)

​

3. send packets to target from compromised hosts

65 of 88

Lines of defense:

Introduction: 1-65

  • authentication: proving you are who you say you are
    • cellular networks provides hardware identity via SIM card; no such hardware assist in traditional Internet
  • confidentiality: via encryption
  • integrity checks: digital signatures prevent/detect tampering
  • access restrictions: password-protected VPNs
  • firewalls: specialized “middleboxes” in access and core networks:
      • off-by-default: filter incoming packets to restrict senders, receivers, applications
      • detecting/reacting to DOS attacks

​

… lots more on security (throughout, Chapter 8)

66 of 88

Chapter 1: roadmap

  • What is the Internet?
  • What is a protocol?
  • Network edge: hosts, access network, physical media
  • Network core: packet/circuit switching, internet structure
  • Performance: loss, delay, throughput
  • Security
  • Protocol layers, service models
  • History

​

Introduction: 1-66

67 of 88

Protocol “layers” and reference models

Introduction: 1-67

Networks are complex,

with many “pieces”:

    • hosts
    • routers
    • links of various media
    • applications
    • protocols
    • hardware, software

​

Question: is there any hope of organizing structure of network?

  • and/or our discussion of networks?

68 of 88

Example: organization of air travel

Introduction: 1-68

  • a series of steps, involving many services

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

How would you define/discuss the system of airline travel?

end-to-end transfer of person plus baggage

69 of 88

Example: organization of air travel

Introduction: 1-69

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

ticketing service

baggage service

gate service

runway service

routing service

layers: each layer implements a service

    • via its own internal-layer actions
    • relying on services provided by layer below

​

70 of 88

Why layering?

Introduction: 1-70

Approach to designing/discussing complex systems:

  • explicit structure allows identification, relationship of system’s pieces
    • layered reference model for discussion
  • modularization eases maintenance, updating of system
    • change in layer's service implementation: transparent to rest of system
    • e.g., change in gate procedure doesn’t affect rest of system

71 of 88

Layered Internet protocol stack

Introduction: 1-71

  • application: supporting network applications
    • HTTP, IMAP, SMTP, DNS
  • 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
    • Ethernet, 802.11 (WiFi), PPP
  • physical: bits “on the wire”

link

application

network

transport

physical

application

transport

network

link

physical

72 of 88

Services, Layering and Encapsulation

Introduction: 1-72

source

  • transport-layer protocol encapsulates application-layer message, M, with transport layer-layer header Ht to create a transport-layer segment
    • Ht used by transport layer protocol to implement its service

application

transport

network

link

physical

destination

application

transport

network

link

physical

Transport-layer protocol transfers M (e.g., reliably) from one process to another, using services of network layer

Ht

M

Application exchanges messages to implement some application service using services of transport layer

M

73 of 88

Services, Layering and Encapsulation

Introduction: 1-73

source

Transport-layer protocol transfers M (e.g., reliably) from one process to another, using services of network layer

Ht

M

  • network-layer protocol encapsulates transport-layer segment [Ht | M] with network layer-layer header Hn to create a network-layer datagram
    • Hn used by network layer protocol to implement its service

application

transport

network

link

physical

destination

M

application

transport

network

link

physical

M

Ht

Hn

Network-layer protocol transfers transport-layer segment [Ht | M] from one host to another, using link layer services

74 of 88

Services, Layering and Encapsulation

Introduction: 1-74

source

Ht

M

  • link-layer protocol encapsulates network datagram [Hn| [Ht |M], with link-layer header Hl to create a link-layer frame

application

transport

network

link

physical

destination

M

application

transport

network

link

physical

M

Ht

Hn

Link-layer protocol transfers datagram [Hn| [Ht |M] from host to neighboring host, using network-layer services

M

Ht

Hn

Hl

M

Ht

Hn

Network-layer protocol transfers transport-layer segment [Ht | M] from one host to another, using link layer services

75 of 88

Encapsulation

Introduction: 1-75

message

segment

datagram

frame

Matryoshka dolls (stacking dolls)

Credit: https://dribbble.com/shots/7182188-Babushka-Boi

76 of 88

Services, Layering and Encapsulation

Introduction: 1-76

source

application

transport

network

link

physical

destination

application

transport

network

link

physical

M

Ht

Hn

Hl

M

Ht

Hn

Ht

M

M

M

message

Ht

M

segment

M

Ht

Hn

datagram

frame

M

Ht

Hn

Hl

77 of 88

Encapsulation: an end-end view

Introduction: 1-77

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

source

Ht

Hn

M

segment

Ht

datagram

destination

Ht

Hn

Hl

M

Ht

Hn

M

Ht

M

M

Ht

Hn

Hl

M

Ht

Hn

M

Ht

Hn

M

Ht

Hn

Hl

M

router

switch

message

M

Ht

M

Hn

frame

link

physical

78 of 88

Chapter 1: roadmap

  • What is the Internet?
  • What is a protocol?
  • Network edge: hosts, access network, physical media
  • Network core: packet/circuit switching, internet structure
  • Performance: loss, delay, throughput
  • Security
  • Protocol layers, service models
  • History

​

Introduction: 1-78

79 of 88

Internet history

  • 1961: Kleinrock - queueing theory shows effectiveness of packet-switching
  • 1964: Baran - packet-switching in military nets
  • 1967: ARPAnet conceived by Advanced Research Projects Agency
  • 1969: first ARPAnet node operational
  • 1972:
    • ARPAnet public demo
    • NCP (Network Control Protocol) first host-host protocol
    • first e-mail program
    • ARPAnet has 15 nodes

​

1961-1972: Early packet-switching principles

80 of 88

Internet history

Introduction: 1-80

  • 1970: ALOHAnet satellite network in Hawaii
  • 1974: Cerf and Kahn - architecture for interconnecting networks
  • 1976: Ethernet at Xerox PARC
  • late70’s: proprietary architectures: DECnet, SNA, XNA
  • 1979: ARPAnet has 200 nodes

1972-1980: Internetworking, new and proprietary networks

Cerf and Kahn’s internetworking principles:

    • minimalism, autonomy - no internal changes required to interconnect networks
    • best-effort service model
    • stateless routing
    • decentralized control

define today’s Internet architecture

81 of 88

Internet history

Introduction: 1-81

  • 1983: deployment of TCP/IP
  • 1982: smtp e-mail protocol defined
  • 1983: DNS defined for name-to-IP-address translation
  • 1985: ftp protocol defined
  • 1988: TCP congestion control
  • new national networks: CSnet, BITnet, NSFnet, Minitel
  • 100,000 hosts connected to confederation of networks

​

1980-1990: new protocols, a proliferation of networks

82 of 88

Internet history

Introduction: 1-82

  • early 1990s: ARPAnet decommissioned
  • 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)
  • early 1990s: Web
    • hypertext [Bush 1945, Nelson 1960’s]
    • HTML, HTTP: Berners-Lee
    • 1994: Mosaic, later Netscape
    • late 1990s: commercialization of the Web

late 1990s – 2000s:

  • more killer apps: instant messaging, P2P file sharing
  • network security to forefront
  • est. 50 million host, 100 million+ users
  • backbone links running at Gbps

​

1990, 2000s: commercialization, the Web, new applications

83 of 88

Internet history

Introduction: 1-83

    • aggressive deployment of broadband home access (10-100’s Mbps)
  • 2008: software-defined networking (SDN)
  • increasing ubiquity of high-speed wireless access: 4G/5G, WiFi
  • service providers (Google, FB, Microsoft) create their own networks
      • bypass commercial Internet to connect “close” to end user, providing “instantaneous” access to social media, search, video content, …
  • enterprises run their services in “cloud” (e.g., Amazon Web Services, Microsoft Azure)
  • rise of smartphones: more mobile than fixed devices on Internet (2017)
  • ~15B devices attached to Internet (2023, statista.com)

​

2005-present: scale, SDN, mobility, cloud

84 of 88

Chapter 1: summary

Introduction: 1-84

We’ve covered a “ton” of material!

  • Internet overview
  • what’s a protocol?
  • network edge, access network, core
    • packet-switching versus circuit-switching
    • Internet structure
  • performance: loss, delay, throughput
  • layering, service models
  • security
  • history

You now have:

  • context, overview, vocabulary, “feel” of networking
  • more depth, detail, and fun to follow!

85 of 88

Additional Chapter 1 slides

Introduction: 1-85

86 of 88

ISO/OSI reference model

Introduction: 1-86

Two layers not found in Internet protocol stack!

  • presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions
  • session: synchronization, checkpointing, recovery of data exchange
  • Internet stack “missing” these layers!
    • these services, if needed, must be implemented in application
    • needed?

application

​

presentation

​

session

​

transport

​

network

​

link

​

physical

The seven layer OSI/ISO

reference model

87 of 88

Services, Layering and Encapsulation

Introduction: 1-87

source

application

transport

network

link

physical

destination

application

transport

network

link

physical

Ht

M

M

M

Ht

Hn

M

Ht

Hn

Hl

M

Ht

Hn

Ht

M

M

message

segment

datagram

frame

M

Ht

Hn

Hl

88 of 88

Wireshark

Introduction: 1-88

Transport (TCP/UDP)

Network (IP)

Link (Ethernet)

Physical

application

(www browser,

email client)

application

OS

packet

capture

(pcap)

packet

analyzer

copy of all Ethernet frames sent/received