Alfaisal University

College of Engineering
EE 305 Computer Networks

Chapter 1
Introduction
Instructor: Driss BH / S. Guizani
Fall: 2025

Slides adapted from Kurose and Ross notes provided

through the publisher. The author’s and publisher’s
copyright holds throughout.
Chapter 1: introduction
Chapter goal: Overview/roadmap:
▪ Get “feel,” “big picture,” ▪ What is the Internet? What is a
introduction to terminology protocol?
• more depth, detail later in ▪ Network edge: hosts, access network,
physical media
course
▪ Network core: packet/circuit switching,
internet structure
▪ Performance: loss, delay, throughput
▪ Protocol layers, service models
▪ Security
▪ History

Introduction: 1-2
The Internet: a “nuts and bolts” view
Billions of connected mobile network
computing devices: national or global ISP
▪ hosts = end systems
▪ running network apps at
Internet’s “edge”

Packet switches: forward

local or
packets (chunks of data) Internet
regional ISP
▪ routers, switches
home network content
Communication links provider
network datacenter
▪ fiber, copper, radio, satellite network

▪ transmission rate: bandwidth

Networks enterprise
▪ collection of devices, routers, network
links: managed by an organization
Introduction: 1-3
“Fun” Internet-connected devices
Tweet-a-watt:
monitor energy use

bikes

Pacemaker & Monitor

Amazon Echo Web-enabled toaster +

IP picture frame
weather forecaster
Internet
refrigerator
Slingbox: remote cars
control cable TV
Security Camera
AR devices
sensorized, scooters
bed
mattress Fitbit

Gaming devices
Others?
Internet phones diapers
Introduction: 1-4
The Internet: a “nuts and bolts” view
mobile network
4G
▪ Internet: “network of networks” national or global ISP

• Interconnected ISPs
▪ protocols are everywhere Skype
IP
Streaming
video
• control sending, receiving of
messages local or
regional ISP
• e.g., HTTP (Web), streaming video,
Skype, TCP, IP, WiFi, 4/5G, Ethernet home network content
provider
HTTP network datacenter
▪ Internet standards network
Ethernet
• RFC: Request for Comments
• IETF: Internet Engineering Task enterprise
TCP

Force network

WiFi
Introduction: 1-5
The Internet: a “services” view
▪ Infrastructure that provides mobile network

services to applications: national or global ISP

• Web, streaming video, multimedia

teleconferencing, email, games, e- Streaming
commerce, social media, inter- Skype video
connected appliances, … local or
regional ISP
▪ provides programming interface
to distributed applications: home network content
provider
• “hooks” allowing sending/receiving HTTP network datacenter
network
apps to “connect” to, use Internet
transport service
• provides service options, analogous enterprise
to postal service network

Introduction: 1-6
What’s a protocol?
Human protocols: Network protocols:
▪ “what’s the time?” ▪ computers (devices) rather than humans
▪ “I have a question” ▪ all communication activity in Internet
▪ introductions governed by protocols

Rules for:
Protocols define the format, order of
… specific messages sent messages sent and received among
… specific actions taken network entities, and actions taken
when message received,
or other events on message transmission, receipt

Introduction: 1-7
What’s a protocol?
A human protocol and a computer network protocol:

Hi TCP connection
request
Hi TCP connection
response
Got the
time? GET http://gaia.cs.umass.edu/kurose_ross
2:00
<file>
time

Q: other human protocols?

Introduction: 1-8
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
Internet history
1961-1972: Early packet-switching principles
▪ 1961: Kleinrock - queueing ▪ 1972:
theory shows effectiveness of • ARPAnet public demo
packet-switching • NCP (Network Control Protocol)
▪ 1964: Baran - packet-switching first host-host protocol
in military nets • first e-mail program
▪ 1967: ARPAnet conceived by • ARPAnet has 15 nodes
Advanced Research Projects
Agency
▪ 1969: first ARPAnet node
operational
Internet history
1972-1980: Internetworking, new and proprietary networks
▪ 1970: ALOHAnet satellite
Cerf and Kahn’s internetworking
network in Hawaii principles:
▪ 1974: Cerf and Kahn - ▪ minimalism, autonomy - no
architecture for interconnecting internal changes required to
networks interconnect networks
▪ best-effort service model
▪ 1976: Ethernet at Xerox PARC ▪ stateless routing
▪ late70’s: proprietary ▪ decentralized control
architectures: DECnet, SNA, XNA define today’s Internet architecture
▪ 1979: ARPAnet has 200 nodes
Introduction: 1-11
Internet history
1980-1990: new protocols, a proliferation of networks
▪ 1983: deployment of TCP/IP ▪ new national networks: CSnet,
▪ 1982: smtp e-mail protocol BITnet, NSFnet, Minitel
defined ▪ 100,000 hosts connected to
▪ 1983: DNS defined for name- confederation of networks
to-IP-address translation
▪ 1985: ftp protocol defined
▪ 1988: TCP congestion control

Introduction: 1-12
Internet history
1990, 2000s: commercialization, the Web, new applications
▪ early 1990s: ARPAnet late 1990s – 2000s:
decommissioned ▪ more killer apps: instant
▪ 1991: NSF lifts restrictions on messaging, P2P file sharing
commercial use of NSFnet ▪ network security to forefront
(decommissioned, 1995)
▪ est. 50 million host, 100 million+
▪ early 1990s: Web users
• hypertext [Bush 1945, Nelson 1960’s]
• HTML, HTTP: Berners-Lee ▪ backbone links running at Gbps
• 1994: Mosaic, later Netscape
• late 1990s: commercialization of the
Web
Introduction: 1-13
Internet history
2005-present: scale, SDN, mobility, cloud
▪ 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)
Introduction: 1-14
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: 1-15
A closer look at Internet structure
mobile network

Network edge: national or global ISP

▪ hosts: clients and servers

▪ servers often in data centers
local or
regional ISP

home network content

provider
network datacenter
network

enterprise
network

Introduction: 1-16
A closer look at Internet structure
mobile network

Network edge: national or global ISP

▪ hosts: clients and servers

▪ servers often in data centers
local or
Access networks, physical media: regional ISP

▪wired, wireless communication links home network content

provider
network datacenter
network

enterprise
network

Introduction: 1-17
A closer look at Internet structure
mobile network

Network edge: national or global ISP

▪ hosts: clients and servers

▪ servers often in data centers
local or
Access networks, physical media: regional ISP

▪wired, wireless communication links home network content

provider
network datacenter

Network core: network

▪ interconnected routers
▪ network of networks enterprise
network

Introduction: 1-18
Access networks and physical media
Q: How to connect end systems mobile network
national or global ISP
to edge router?
▪ residential access nets
▪ institutional access networks (school,
company)
local or
▪ mobile access networks (WiFi, 4G/5G) regional ISP

home network content

provider
network datacenter
network

enterprise
network

Introduction: 1-19
Access networks: cable-based access
cable headend

cable splitter
modem

C
O
V V V V V V N
I I I I I I D D T
D D D D D D A A R
E E E E E E T T O
O O O O O O A A L

1 2 3 4 5 6 7 8 9

Channels

frequency division multiplexing (FDM): different channels transmitted in

different frequency bands
Introduction: 1-20
Access networks: cable-based access
cable headend

cable splitter cable modem

modem CMTS termination system
data, TV transmitted at different
frequencies over shared cable ISP
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
Introduction: 1-21
Access networks: digital subscriber line (DSL)
central office telephone
network

DSL splitter
modem DSLAM

voice, data transmitted ISP

at different frequencies over DSL access
dedicated line to central office multiplexer

▪ 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
Introduction: 1-22
Access networks: home networks
Wireless and wired
devices

to/from headend or
central office
often combined
in single box

cable or DSL modem

WiFi wireless access router, firewall, NAT

point (54, 450 Mbps)
wired Ethernet (1 Gbps)
Introduction: 1-23
Wireless access networks
Shared wireless access network connects end system to router
▪ via base station aka “access point”

Wireless local area networks Wide-area cellular access networks

(WLANs) ▪ provided by mobile, cellular network
▪ typically within or around operator (10’s km)
building (~100 ft) ▪ 10’s Mbps
▪ 802.11b/g/n (WiFi): 11, 54, 450 ▪ 4G/5G cellular networks
Mbps transmission rate

to Internet
to Internet
Introduction: 1-24
Access networks: enterprise networks

Enterprise link to
ISP (Internet)
institutional router
Ethernet institutional mail,
switch web servers

▪ 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
Introduction: 1-25
Access networks: data center networks
mobile network
▪ high-bandwidth links (10s to 100s national or global ISP
Gbps) connect hundreds to thousands
of servers together, and to Internet

local or
regional ISP

home network content

provider
network datacenter
network

Courtesy: Massachusetts Green High Performance Computing enterprise

Center (mghpcc.org) network

Introduction: 1-26
Host: sends packets of data
host sending function:
▪ takes application message
▪ breaks into smaller chunks, two packets,
known as packets, of length L bits L bits each

▪ transmits packet into access

2 1
network at transmission rate R
• link transmission rate, aka link host
capacity, aka link bandwidth R: link transmission rate

packet time needed to L (bits)

transmission = transmit L-bit =
delay packet into link R (bits/sec)
Introduction: 1-27
Links: physical media
▪ bit: propagates between Twisted pair (TP)
transmitter/receiver pairs
▪ two insulated copper wires
▪ physical link: what lies • Category 5: 100 Mbps, 1 Gbps Ethernet
between transmitter & • Category 6: 10Gbps Ethernet
receiver
▪ guided media:
• signals propagate in solid
media: copper, fiber, coax
▪ unguided media:
• signals propagate freely,
e.g., radio

Introduction: 1-28
Links: physical media
Coaxial cable: Fiber optic cable:
▪ two concentric copper conductors ▪ glass fiber carrying light pulses, each
pulse a bit
▪ bidirectional
▪ high-speed operation:
▪ broadband: • high-speed point-to-point
• multiple frequency channels on cable transmission (10’s-100’s Gbps)
• 100’s Mbps per channel ▪ low error rate:
• repeaters spaced far apart
• immune to electromagnetic noise

Introduction: 1-29
Links: physical media
Wireless radio Radio link types:
▪ signal carried in various ▪ Wireless LAN (WiFi)
“bands” in electromagnetic • 10-100’s Mbps; 10’s of meters
spectrum ▪ wide-area (e.g., 4G/5G cellular)
▪ no physical “wire” • 10’s Mbps (4G) over ~10 Km
▪ broadcast, “half-duplex” ▪ Bluetooth: cable replacement
(sender to receiver)
• short distances, limited rates
▪ propagation environment
effects: ▪ terrestrial microwave
• reflection • point-to-point; 45 Mbps channels
• obstruction by objects ▪ satellite
• Interference/noise • up to < 100 Mbps (Starlink) downlink
• 270 msec end-end delay (geostationary)
Introduction: 1-30
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: 1-31
The network core
▪ mesh of interconnected routers mobile network
national or global ISP
▪ packet-switching: hosts break
application-layer messages into
packets
• network forwards packets from one local or
regional ISP
router to the next, across links on
path from source to destination home network content
provider
network datacenter
network

enterprise
network

Introduction: 1-32
 Two key network-core functions

routing algorithm Routing:

Forwarding: local
local forwarding
forwarding table
table
▪ global action:
header value output link determine source-
▪ aka “switching” 0100
0101
3
2 destination paths
▪ local action: 0111 2
taken by packets
1001 1
move arriving
packets from ▪ routing algorithms
router’s input link 1
to appropriate
router output link 3 2

destination address in arriving

packet’s header
Introduction: 1-33
routing

Introduction: 1-34
 forwarding
forwarding

Introduction: 1-35
 Packet-switching: store-and-forward

L bits
per packet
3 2 1
source destination
R bps R bps

▪ packet transmission delay: takes L/R seconds to One-hop numerical example:

transmit (push out) L-bit packet into link at R bps ▪ L = 10 Kbits
▪ store and forward: entire packet must arrive at ▪ R = 100 Mbps
router before it can be transmitted on next link ▪ one-hop transmission delay
= 0.1 msec

Introduction: 1-36
Packet-switching: queueing
R = 100 Mb/s
A C

D
B R = 1.5 Mb/s
E
queue of packets
waiting for transmission
over output link

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

Introduction: 1-37
Packet-switching: queueing
R = 100 Mb/s
A C

D
B R = 1.5 Mb/s
E
queue of packets
waiting for transmission
over output link

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: 1-38
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)
▪ commonly used in traditional telephone networks

* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive
Introduction: 1-39
Circuit switching: FDM and TDM
Frequency Division Multiplexing
(FDM) 4 users

frequency
▪ optical, electromagnetic frequencies
divided into (narrow) frequency
bands
▪ each call allocated its own band, can
transmit at max rate of that narrow time
band

Time Division Multiplexing (TDM)

frequency
▪ time divided into slots
▪ each call allocated periodic slot(s), can
transmit at maximum rate of (wider) time
frequency band (only) during its time
slot(s) Introduction: 1-40
Packet switching versus circuit switching
example:
▪ 1 Gb/s link
N
▪ each user: users 1 Gbps link
• 100 Mb/s when “active”
• active 10% of time

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

▪ circuit-switching: 10 users
▪ packet switching: with 35 users, Q: how did we get value 0.0004?
probability > 10 active at same time
is less than .0004 *
A: HW problem (for those with
course in probability only)

* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive
Introduction: 1-41
Packet switching versus circuit switching
Is packet switching a “slam dunk winner”?
▪ 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.

Q: human analogies of reserved resources (circuit switching) versus

on-demand allocation (packet switching)?
Introduction: 1-42
Internet structure: a “network of networks”
mobile network
▪ hosts connect to Internet via access national or global ISP
Internet Service Providers (ISPs)
▪ access ISPs in turn must be
interconnected
• so that any two hosts (anywhere!) local or
regional ISP
can send packets to each other
▪ resulting network of networks is home network content
provider
very complex network datacenter
network

• evolution driven by economics, enterprise

national policies network

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

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

access access
net net

access
net
access
net

access
net
access
net
access access
net access net
net

Introduction: 1-44
Internet structure: a “network of networks”
Question: given millions of access ISPs, how to connect them together?
access access
net net
access
net
access
access net
net
access
access net
net

connecting each access ISP to

each other directly doesn’t scale:
access
access
net O(N2) connections. net

access
net
access
net

access
net
access
net
access access
net access net
net

Introduction: 1-45
Internet structure: a “network of networks”
Option: connect each access ISP to one global transit ISP?
Customer and provider ISPs have economic agreement.
access access
net net
access
net
access
access net
net
access
access net
net

global
access
net
ISP access
net

access
net
access
net

access
net
access
net
access access
net access net
net

Introduction: 1-46
Internet structure: a “network of networks”
But if one global ISP is viable business, there will be competitors ….

access access
net net
access
net
access
access net
net
access
access net
net ISP A

access
net ISP B access
net

access ISP C
net
access
net

access
net
access
net
access access
net access net
net

Introduction: 1-47
Internet structure: a “network of networks”
But if one global ISP is viable business, there will be competitors …. who will
want to be connected
Internet exchange point
access access
net net
access
net
access
access net
net
IXP access
access net
net ISP A

access
net
IXP ISP B access
net

access ISP C
net
access
net

access
net
peering link
access
net
access access
net access net
net

Introduction: 1-48
Internet structure: a “network of networks”
… and regional networks may arise to connect access nets to ISPs

access access
net net
access
net
access
access net
net
IXP access
access net
net ISP A

access
net
IXP ISP B access
net

access ISP C
net
access
net

access
net
regional ISP access
net
access access
net access net
net

Introduction: 1-49
Internet structure: a “network of networks”
… and content provider networks (e.g., Google, Microsoft, Akamai) may
run their own network, to bring services, content close to end users
access access
net net
access
net
access
access net
net
IXP access
access net
net ISP A

Content provider network

access
net
IXP ISP B access
net

access ISP C
net
access
net

access
net
regional ISP access
net
access access
net access net
net

Introduction: 1-50
 Internet structure: a “network of networks”
Tier 1 ISP Tier 1 ISP Google
IXP IXP IXP
Regional ISP Regional ISP

access access access access access access access access

ISP ISP ISP ISP ISP ISP ISP 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
Introduction: 1-51
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: 1-52
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
packet being transmitted (transmission delay)

B
packets in buffers (queueing delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Introduction: 1-53
Packet delay: four sources
transmission
A propagation

B
nodal
processing queueing

dnodal = dproc + dqueue + dtrans + dprop

dproc: nodal processing dqueue: queueing delay

▪ check bit errors ▪ time waiting at output link for
▪ determine output link transmission
▪ typically < microsecs ▪ depends on congestion level of
router
Introduction: 1-54
Packet delay: four sources
transmission
A propagation

B
nodal
processing queueing

dnodal = dproc + dqueue + dtrans + dprop

dtrans: transmission delay: dprop: propagation delay:
▪ L: packet length (bits) ▪ d: length of physical link
▪ R: link transmission rate (bps) ▪ s: propagation speed (~2x108 m/sec)
▪ dtrans = L/R ▪ dprop = d/s
dtrans and dprop
very different
Introduction: 1-55
Caravan analogy
100 km 100 km

ten-car caravan toll booth toll booth toll booth

(aka 10-bit packet) (aka link)

▪ car ~ bit; caravan ~ packet; toll ▪ time to “push” entire caravan

service ~ link transmission through toll booth onto
▪ toll booth takes 12 sec to service highway = 12*10 = 120 sec
car (bit transmission time) ▪ time for last car to propagate
▪ “propagate” at 100 km/hr from 1st to 2nd toll both:
100km/(100km/hr) = 1 hr
▪ Q: How long until caravan is lined
up before 2nd toll booth? ▪ A: 62 minutes

Introduction: 1-56
Caravan analogy
100 km 100 km

ten-car caravan toll booth toll booth

(aka 10-bit packet) (aka router)

▪ 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

Introduction: 1-57
Packet queueing delay (revisited)
▪ a: average packet arrival rate

average queueing delay

▪ L: packet length (bits)
▪ R: link bandwidth (bit transmission rate)

L .a arrival rate of bits “traffic

:
R service rate of bits intensity” traffic intensity = La/R 1

▪ La/R ~ 0: avg. queueing delay small La/R ~ 0

▪ La/R -> 1: avg. queueing delay large

▪ La/R > 1: more “work” arriving is
more than can be serviced - average
delay infinite!
La/R -> 1
Introduction: 1-58
“Real” Internet delays and routes
▪ 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:
• 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

3 probes 3 probes

3 probes

Introduction: 1-59
Real Internet delays and routes
traceroute: gaia.cs.umass.edu to www.eurecom.fr
3 delay measurements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 3 delay measurements
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 to border1-rt-fa5-1-0.gw.umass.edu
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 trans-oceanic link
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 looks like delays
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 decrease! Why?
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 * * * * means no response (probe lost, router not replying)
19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms

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

Introduction: 1-60
Packet loss
▪ queue (aka buffer) preceding link in buffer has finite capacity
▪ packet arriving to full queue dropped (aka lost)
▪ lost packet may be retransmitted by previous node, by source end
system, or not at all
buffer
(waiting area) packet being transmitted
A

B
packet arriving to
full buffer is lost

* Check out the Java applet for an interactive animation (on publisher’s website) of queuing and loss
Introduction: 1-61
 Throughput
▪ 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

link capacity
pipe that can carry linkthat
pipe capacity
can carry
Rsfluid
bits/sec
at rate Rfluid
c bits/sec
at rate
serverserver,
sends with
bits
(fluid) into pipe (Rs bits/sec) (Rc bits/sec)
file of F bits
to send to client
Introduction: 1-62
Throughput
Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

bottleneck link
link on end-end path that constrains end-end throughput
Introduction: 1-63
Throughput: network scenario
▪ per-connection end-
Rs end throughput:
Rs Rs min(Rc,Rs,R/10)
▪ in practice: Rc or Rs is
R often bottleneck
Rc Rc
Rc
* Check out the online interactive exercises for more
examples: http://gaia.cs.umass.edu/kurose_ross/

10 connections (fairly) share

backbone bottleneck link R bits/sec
Introduction: 1-64
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: 1-65
Protocol “layers” and reference models
Networks are complex, Question: is there any
with many “pieces”: hope of organizing
▪ hosts structure of network?
▪ routers ▪and/or our discussion
▪ links of various media of networks?
▪ applications
▪ protocols
▪ hardware, software

Introduction: 1-66
Example: organization of air travel
end-to-end transfer of person plus baggage
ticket (purchase) ticket (complain)
baggage (check) baggage (claim)
gates (load) gates (unload)
runway takeoff runway landing
airplane routing airplane routing
airplane routing

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

▪ a series of steps, involving many services
Introduction: 1-67
Example: organization of air travel

ticket (purchase) ticketing service ticket (complain)

baggage (check) baggage service baggage (claim)
gates (load) gate service gates (unload)
runway takeoff runway service runway landing
airplane routing routing service
airplane routing airplane routing

layers: each layer implements a service

▪ via its own internal-layer actions
▪ relying on services provided by layer below
Introduction: 1-68
Why layering?
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

Introduction: 1-69
Layered Internet protocol stack
▪ application: supporting network applications
• HTTP, IMAP, SMTP, DNS
application
application
▪ transport: process-process data transfer
• TCP, UDP transport
transport
▪ network: routing of datagrams from source to
destination network
• IP, routing protocols
link
▪ link: data transfer between neighboring
network elements physical
• Ethernet, 802.11 (WiFi), PPP
▪ physical: bits “on the wire”
Introduction: 1-70
 Services, Layering and Encapsulation
M
application Application exchanges messages to implement some application
application service using services of transport layer
Ht M
transport Transport-layer protocol transfers M (e.g., reliably) from transport
one process to another, using services of network layer

network ▪ transport-layer protocol encapsulates network

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

source destination

Introduction: 1-71
 Services, Layering and Encapsulation
M
application application
Ht M
transport Transport-layer protocol transfers M (e.g., reliably) from transport
one process to another, using services of network layer

network Hn Ht M network
Network-layer protocol transfers transport-layer segment
[Ht | M] from one host to another, using link layer services
link link
▪ network-layer protocol encapsulates
transport-layer segment [Ht | M] with
physical network layer-layer header Hn to create a physical
network-layer datagram
source • Hn used by network layer protocol to destination
implement its service
Introduction: 1-72
 Services, Layering and Encapsulation
M
application application
Ht M
transport transport

network Hn Ht M network
Network-layer protocol transfers transport-layer segment
[Ht | M] from one host to another, using link layer services
link Hl Hn Ht M link
Link-layer protocol transfers datagram [Hn| [Ht |M] from
host to neighboring host, using network-layer services
physical physical
▪ link-layer protocol encapsulates network
datagram [Hn| [Ht |M], with link-layer header
source Hl to create a link-layer frame destination

Introduction: 1-73
 Encapsulation
Matryoshka dolls (stacking dolls)

message segment datagram frame

Credit: https://dribbble.com/shots/7182188-Babushka-Boi Introduction: 1-74

 Services, Layering and Encapsulation

application message M M application

transport segment Ht M Ht M transport

datagram Hn Ht M
network Hn Ht M network

link frame Hl Hn Ht M Hl Hn Ht M link

physical physical

source destination

Introduction: 1-75
 message M
source
application
Encapsulation: an
segment
datagram Hn Ht
Ht M
M
transport
network
end-end view
frame Hl Hn Ht M link
physical
link
physical

switch

destination Hn Ht M network
M application Hl Hn Ht M link Hn Ht M
Ht M transport physical
Hn Ht M network
Hl Hn Ht M link router
physical
Introduction: 1-76
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: 1-77
Network security
▪ 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
Introduction: 1-78
Network security
▪ 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
Introduction: 1-79
Bad guys: packet interception
packet “sniffing”:
▪ broadcast media (shared Ethernet, wireless)
▪ promiscuous network interface reads/records all packets (e.g.,
including passwords!) passing by

A C

src:B dest:A payload

B

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

Introduction: 1-80
Bad guys: fake identity
IP spoofing: injection of packet with false source address

A C

src:B dest:A payload

Introduction: 1-81
Bad guys: denial of service
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 target

from compromised
hosts
Introduction: 1-82
Lines of defense:
▪ 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) Introduction: 1-83

Chapter 1: summary
We’ve covered a “ton” of material!
▪ Internet overview
▪ what’s a protocol? You now have:
▪ network edge, access network, core ▪ context, overview,
• packet-switching versus circuit-
switching vocabulary, “feel”
• Internet structure of networking
▪ performance: loss, delay, throughput ▪ more depth,
▪ layering, service models detail, and fun to
▪ security follow!
▪ history
Introduction: 1-84
Additional Chapter 1 slides

Introduction: 1-85
 ISO/OSI reference model
Two layers not found in Internet
application
protocol stack!
presentation
▪ presentation: allow applications to
interpret meaning of data, e.g., encryption, session
compression, machine-specific conventions transport
▪ session: synchronization, checkpointing, network
recovery of data exchange link
▪ Internet stack “missing” these layers! physical
• these services, if needed, must be
implemented in application The seven layer OSI/ISO
reference model
• needed?
Introduction: 1-86
More than seven OSI layers

Introduction: 1-87
 Services, Layering and Encapsulation
M
application M application
message
Ht M
transport Ht M transport
segment
network Hn Ht M Hn Ht M network
datagram

link Hl Hn Ht M Hl Hn Ht M link
frame

physical physical

source destination

Introduction: 1-88
Wireshark
application
(www browser,
packet
email client)
analyzer
application

OS
packet Transport (TCP/UDP)
Network (IP)
capture copy of all
Ethernet frames Link (Ethernet)
(pcap) sent/received
Physical

Introduction: 1-89