Data Networks
Introduction 1-1
Data Networks
Organization:
• Core lecture, 9 ECTS points
• Lectures: Tue 10-12 (Günter-Hotz lecture hall), Thu 1214 (HS002 except for 25th)
• Tutorial: Fri 12-14 (HS002, except for May 31st , June 21st)
• Written exam (25.07), re-exam (beginning of October)
• Weekly quizzes on Wednesdays: 50% of points to
participate in exam
• Weekly exercises (optional): solutions are discussed in
tutorial (sheets are uploaded one week earlier)
Data Networks
Organization:
• Questions about the organization: {turrini, spieler,
mikeev}@cs.uni-…
• Course webpage: mosi.cs.uni-saarland.de -> teaching
• you may post live comments during the lecture:
• everything you want to discuss immediately without
interrupting the presentation
• you may also post interesting links related to the
material presented
• feedback concerning the presentation
• forum for “dead (not live)” comments/questions
Data Networks
Literature:
• Computer Networking by J.F Kurose and
K.W. Ross, 6th edition (Chapter 1-6)
(=> Bock & Seip store on campus!)
• Data Networks by Dimitri P. Bertsekas and
Robert G. Gallager, 2nd edition (Chapter 3)
Syllabus
• 1: Introduction: Broad overview of computer networking and the
Internet
• 2: Application Layer: HTTP, FTP, SMTP, DNS, P2P, …
• 3: Transport Layer: Multiplexing, TCP, Congestion control, …
• 4: Network Layer: IP and routing, …
• 5: Link Layer: Links, Access Networks, LANs, …
• 6: Wireless Networks
• 7: Delay Models: Little’s Theorem, Queueing Systems, …
Further reading:
• Computer Networking by J.F Kurose and K.W. Ross, 6th edition (Chapter
7-9) Multimedia Networking, Security in Computer Networks, Network
Management
• Computer Networks by A. Tanenbaum, 4th ed
• Computer Networks by L. Peterson, B. Davie, 5th Edition
Syllabus
What this lecture is (not) about:
Fundamental principles of computer networks in general!
Internet is just an instance/ an example where we can see
how things are implemented in practice.
Not: details of most recent developments of the Internet
(currently “in”, may be “out” in two years ...)
Chapter 1: introduction
our goal:
 get broad overview
and terminology
 more depth, detail
later in course
 approach:
 use Internet as
example
overview:








what’s the Internet?
what’s a protocol?
network edge; hosts (servers,
Laptops, smartphones, ...)
network core: packet/circuit
switching, Internet structure
performance: loss, delay,
throughput
security (no details later)
protocol layers
history
Introduction 1-7
Chapter 1: roadmap
1.1 what is the Internet?
1.2 network edge
1.3 network core
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 networks under attack: security
1.7 history
Introduction 1-8
What’s the Internet: “nuts and bolts” view
of millions of
server
connected computing
devices:
wireless
laptop
 hosts = end systems
smartphone
 running network apps
 communication links
 coaxial cable,
wireless
links
copper, fiber, radio,
wired
…
links
 transmission rate:
bandwidth
PC
 hundreds
global ISP
home
network
(WiFi)
regional ISP
 Packet
router
switches: receive and
forward packets (chunks of
data)
 routers and switches
mobile network
institutional
network
Introduction 1-9
What’s the Internet: “nuts and bolts” view

Internet Service Provider (ISP):
 End systems access the Internet through
ISPs
 Interconnected ISPs (=> “network of
networks”)
 classification according to size (Tier 3
(local), Tier 2 (national), Tier
1(global/international)

mobile network
global ISP
home
network
regional ISP
protocols control sending, receiving of
msgs
 e.g., TCP, IP, HTTP, SMTP, …

Internet standards
 different components of the internet
have to agree on standards to
interoperate
 IETF: Internet Engineering Task Force
develops and promotes such standards
 IETF publishes RFCs (Requests for
comments) describing/defining protocols
institutional
network
Introduction 1-10
What’s the Internet: a service view
The Internet from a different
perspective:

Infrastructure that provides
services to applications:
 Web, VoIP, email, distributed
games, P2P, social nets,...
 applications are distributed
(involve multiple hosts
exchanging data)

mobile network
global ISP
home
network
regional ISP
End systems provides applic.
programming interface to apps
 rules for exchanging data
between programms
 provides service options,
analogous to postal service
institutional
network
Introduction 1-11
What’s a protocol?
a human analogy:
teacher
Hi
“broadcast”
"Questions?"
Hi
raise hand
Got the
time?
“Yes?"
2:00
ask
time
answer
Introduction 1-12
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://mosi.cs.uni-saarland.de
2:00
<file>
time
Introduction 1-13
What’s a protocol?
network protocols:


communication between machines
rather than humans
all communication activity in Internet
governed by protocols
protocols define format, order of messages sent and
received among network entities, and actions taken
on message transmission or receipt
Introduction 1-14
Chapter 1: roadmap
1.1 what is the Internet?
1.2 network edge
1.3 network core
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 networks under attack: security
1.7 history
just a quick overview; no technical details!
Introduction 1-15
A closer look at network structure:

network edge:



hosts: clients and servers
home-based hosts:
desktop computers,
laptops, smartphones,
tablets
servers often reside in
large data centers (Web,
email)
mobile network
global ISP
home
network
regional ISP
institutional
network
Introduction 1-16
Non-traditional end systems
Web-enabled toaster +
weather forecaster
IP picture frame
http://www.ceiva.com/
Tweet-a-watt:
monitor energy use
Slingbox: watch,
control cable TV remotely
Internet
refrigerator
Internet phones
Introduction 1-17
A closer look at network structure:

network edge:



mobile network
hosts: clients and servers
global ISP
access networks: physical
connection of end system
to first router (“edge
router”); wired or wireless
communication links
home
network
regional ISP
network core:
 interconnected routers
 network of networks
institutional
network
Introduction 1-18
Access networks and physical media
edge router
How to connect end systems
to edge router?
different types



residential access nets
institutional access
networks (school,
company)
mobile access networks
Introduction 1-19
Access net: digital subscriber line (DSL)
• DSL Internet access obtained from local telephone
company => ISP
• Market share in Germany 86% (in 2012)
• use existing telephone line (copper wire, analog
transmission) to DSL access multiplexer (DSLAM) of
phone company
• DSLAM usually located in the central office
(CO)„Vermittlungsstelle“ (or it is an outdoor DSLAM)
• data over DSL line goes to Internet
• voice over DSL line goes to telephone net
Access net: digital subscriber line (DSL)
central office
DSL splitter
modem
voice, data transmitted
at different frequencies over
dedicated line to central office



telephone
network
DSLAM
ISP
DSL access
multiplexer
use different frequencies for telephone and Internet data
=> splitter separates rec. signal & forwards data to DSL modem
DSLAM separates the incoming data/phone signals and translates
it into digital format
rates are either limited by ISP or because of the distance to CO
Introduction 1-21
Access net: cable network
cable headend
…
cable splitter
modem
CMTS
cable modem
termination system
ISP




make use of the cable television infrastructure
residence: needs cable modem to exchange data with cable
modem termination system (CMTS)
splitter often integrated in cable modem
both optical fibre and coaxial cable are employed (HFC: hybrid
fiber coax)
Introduction 1-22
Access net: cable network
cable headend
…
cable splitter
modem
data, TV transmitted at different
frequencies over shared cable
distribution network

CMTS
cable modem
termination system
ISP
network of cable/fiber attaches homes to ISP router
 homes share access network to cable headend
 => transm. rate decreases in case of high traffic
 => requested data packets are sent to all connected
participants of a group
 unlike DSL, which has dedicated access to central office
Introduction 1-23
Access net: home network
wireless
devices
to/from headend or
central office
often combined
in single box
cable or DSL modem
wireless access
point (54 Mbps)
router
wired Ethernet (100 Mbps)
Introduction 1-24
Enterprise access networks (Ethernet)
institutional link to
ISP (Internet)
institutional router
Ethernet
switch


typically used in companies, universities, etc
transmission rates:



institutional mail,
web servers
10 Mbps to 100Mbps (users),
1Gbps, 10Gbps (servers)
today, end systems typically connect into Ethernet switch
Introduction 1-25
Wireless access networks

wireless access network connects end system to router
 via “access point”; uses radio signals to transmit data
 access point shared by several end systems
wireless LANs:
distinguish between
wide-area wireless access
 IEEE 802.11 technology (WiFi)
 up to 54 Mbps transmission rate
 range: typically a few 10 meters
to Internet
maximal bandwidths
 use infrastructure of cellular
telephony
 connect to base stations
 range: few 10’s of km
to Internet
Introduction 1-26
Physical media



bit: sent by propagating
electromagnetic waves (analog
signal) or optical pulses (digital
signal) between
transmitter/receiver pairs
physical link: what lies between
transmitter & receiver
distinguish:
 guided media: signals
propagate in solid media:
copper, fiber-optic, coaxial
 unguided media: signals
propagate freely, e.g., radio
waves
twisted pair (TP) copper wire
 two insulated copper
wires arranged in regular
spiral pattern



least expensive, most
commonly used
10 Mbps to 10 Gpbs
Ethernet, DSL
Introduction 1-27
Physical media: coax, fiber
coaxial cable:


two concentric copper
conductors
used for HFC (hybrid fibre
coaxial) access via (cable
internet)
fiber optic cable:

glass fiber carrying light
pulses (digital signal)

high-speed point-to-point
transmission (10’s-100’s Gpbs)

low error rate (immune to

transmitters, receiver, switches
are costly
long distance link (oversea)
prevalent in Internet backbone


electromagnetic noise)
Introduction 1-28
Physical media: radio
radio link types:

terrestrial radio channels
 short distance (e.g. Bluetooth)
 LAN (e.g., WiFi)
 cellular access (wide–area wireless access)

satellite links






used if DSL or cable is not available
transmission rate:10’s of Mbps
> 270 msec signal propagation delay (DSL has ony 20 ms)
typically using a geostationary satellite
location needs to be in the footprint of the satellite
influenced by meteorological factors
Introduction 1-29
Chapter 1: 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
Introduction 1-30
The network core
mesh of interconnected
routers
 What do routers do?
packet-switching:

 hosts run network apps and
break application-layer
messages into packets
 forward packets from one
router to the next, across
links on path from source to
destination
 packets may be buffered and
queue while passing network
nodes
Introduction 1-31
Host: sends packets of data
host sending function:
 takes application message
 breaks into smaller chunks,
known as packets, of length
L bits (packetization)
 transmits packet into access
network at transmission rate
R (bits/sec)
 link transmission rate
= link bandwidth
two packets,
L bits each
2 1
R: link transmission rate
host
How many seconds does it take to transmit one L-bit paket?
packet
transmission
delay
=
time needed to
transmit L-bit
packet into link
=
L (bits)
R (bits/sec)
Host: sends packets of data
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 (bits/sec)
 transmission delay = time
for pushing the packet‘s bits
into the link
 propagation delay =
traveling across the wire
two packets,
L bits each
2 1
R: link transmission rate
host
Packet-switching: store-and-forward mechanism
L bits
per packet
source



3 2 1
R bps
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
end-end delay (two links with
bandwidth R)= 2L/R (assuming
zero propagation delay)
R bps
destination
one-hop numerical example:
 L = 7.5 Mbits
 R = 1.5 Mbps
 one-hop transmission
delay = 5 sec
more on delay shortly …
Introduction 1-34
Packet-switching: store-and-forward
L bits
per packet
source
3 2 1
R bps
R bps
destination
takes L/R seconds to
 Question:
transmit (push out) L-bit
What is the end-end delay if a
packet into link at R bps
packet is sent via N links (and N-1)
 store and forward: entire
routers?
packet must arrive at router
before it can be transmitted
N x L/R
on next link
 end-end delay = 2L/R (assuming
zero propagation delay)

Introduction 1-35
Packet Switching: queueing delay, loss
queuing and loss:


routers have one buffer per outgoing link => store packets
that the router is about to send into the link
If arrival rate (in bits) to link exceeds transmission rate of
link for a period of time:
 packets will queue, wait to be transmitted on link
 packets can be dropped (lost) if memory (buffer) fills up
A
B
C
R = 100 Mb/s
R = 1.5 Mb/s
queue of packets
waiting for output link
D
E
Introduction 1-36
Key functions in paket switching
routing: determines sourcedestination route taken by
packets
 routing algorithms
forwarding: move packets from
router’s input to appropriate
router output
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
1
3 2
dest address in arriving
packet’s header
Network Layer 4-37
Paket vs. circuit switching
two fundamental approaches for moving data through a
network:
packets switching and
circuit switching
- in packet-switched networks
resources are NOT reserved
(=> no guarantees!)
restaurant analogy: just go
there without reservation!
- in circuit-switched networks
resources are reserved
- links are separated into
circuit segments
- fraction of each links
transmission capacity is
reserved for the duration of
the connection
restaurant analogy: reserve a
table in advance!
Introduction 1-38
Alternative core: circuit switching
resources (buffers, links,
transmission rate) are reserved
for “call” between source &
destination:




Here, 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)
constant transmission rate
circuit segment idle if not used
by call (no sharing)
no store and forward =>
transmission time independent
of number of links
Commonly used in
traditional telephone
networks
Introduction 1-39
Circuit switching: FDM versus TDM
Example:
Frequency-division multiplexing
4 users
frequency
Time-division multiplexing
time
frequency
time
Introduction 1-40
Packet switching versus circuit switching
packet switching allows more users to use network!
example:
 users share 1000 kb/s link
 each user:
• wants to transmit 100 kb/s
when “active”
• active 10% of time
N
users
1000 kb/s link
 circuit-switching:
 can have at most 10? users
 packet
switching:
 with 35 users, probability >
10 active at same time is less
than .0004
Q: how did we get value 0.0004?
Q: what happens if > 35 users ?
=> 3 times more users at the same performance with prob 99,96 %
Introduction 1-41
Packet switching versus circuit switching
packet switching



great for bursty data
 resource sharing
 simpler, no call setup
excessive congestion possible: packet delay and loss
 protocols needed for reliable data transfer, congestion
control
Problem: How to provide circuit-like behavior?
 bandwidth guarantees needed for audio/video streaming
 still an unsolved problem ...
Introduction 1-42
Internet structure: network of networks


End systems connect to Internet via access ISPs (Internet
Service Providers)
 Residential, company and university ISPs
Access ISPs in turn must be interconnected.
 So that any two hosts can send packets to each other
Let’s now take a stepwise approach to
describe the current Internet structure.
Internet structure: network of networks
Question: given thousands 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
Internet structure: network of networks
Option: connect each access ISP to every other access ISP?
access
net
access
net
access
net
access
net
access
net
access
net
access
net
connecting each access ISP
to each other directly doesn’t
scale: O(N2) connections.
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
Option: connect each access ISP to a single global transit ISP?
Access ISPs (customers) have to pay global ISP for traffic
=> customer and provider ISPs need economic agreement.
access
net
access
net
access
net
access
net
access
net
access
net
access
net
global
ISP
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
But if one global ISP is viable business, there will be competitors
….
access
net
access
net
access
net
access
net
access
net
access
net
access
net
ISP A
access
net
access
net
access
net
ISP B
ISP C
access
net
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
But if one global ISP is viable business, there will be competitors
…. which must be interconnected
Internet exchange point
access
net
(multiple ISPs peer together)
access
net
access
net
access
net
access
net
IXP
access
net
ISP A
IXP
access
net
access
net
access
net
access
net
ISP B
ISP C
access
net
peering link
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
… and regional networks may arise to connect access nets to
ISPS
access
net
access
net
access
net
access
net
access
net
IXP
access
net
ISP A
IXP
access
net
access
net
access
net
access
net
ISP B
ISP C
access
net
access
net
regional net
access
net
access
net
access
net
access
net
Internet structure: network of networks
… and content provider networks (e.g., Google, Microsoft, ...)
may run their own network, to bring services, content close to
end users
access
net
access
net
access
net
access
net
access
net
IXP
access
net
ISP A
access
net
Content provider network
IXP
access
net
access
net
access
net
ISP B
ISP B
access
net
access
net
regional net
access
net
access
net
access
net
access
net
Internet structure: network of networks
at center:
small # of
wellTier 1 ISP
connected
large
networks
Tier 1 ISP
IXP
IXP
Regional ISP
access
ISP
access
ISP
Google
access
ISP
access
ISP
IXP
Regional ISP
access
ISP
access
ISP
access
ISP
access
ISP
 Tier-1 commercial ISPs: national & international coverage, providers of
Internet backbone (backbone is the core of the actual internet)
 content provider network (e.g, Google): private network that connects
it data centers to Internet, often bypassing tier-1, regional ISPs
Introduction 1-51
Chapter 1: 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
Introduction 1-52
How do loss and delay occur?
packets queue in router buffers


packet arrival rate to link (temporarily) exceeds output link
transmission rate
packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Introduction 1-53
Four sources of packet delay
transmission
A
propagation
B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dproc: nodal processing




examine header of packet
check for bit-level errors
determine output link
typically < 1/100 millisec (ms)
dqueue: queueing delay
 time waiting at output link
for transmission
 depends on congestion
level of router
 1/1000 – 1 ms
Introduction 1-54
Four sources of packet delay
transmission
A
propagation
B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
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 meters/sec)
 dprop = d/s
 in wide area networks: order of
milliseconds otherwise negligible
Introduction 1-55
Caravan analogy
100 km
ten-car
caravan





toll
booth
highway
“propagation speed” of cars:
100 km/hr
cars have fixed order
toll booth: on arrival, first car
waits for others; takes 12 sec to
service a car (bit transmission
time)
car~bit; caravan ~ packet
Q: How long until caravan is
lined up before 2nd toll booth?
100 km
toll
booth
highway
 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
Introduction 1-56
Caravan analogy (more)
100 km
ten-car
caravan



toll
booth
100 km
toll
booth
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, 1st car arrives at second booth; three
cars still at 1st booth.
=> this situation arises in packet-switched networks: bits
arrive at the router before the last bit is pushed into the link
at the source router
Introduction 1-57
http://wps.pearsoned.com/ecs_kurose_compnetw_6/216/55463/141
98700.cw/index.html
Introduction 1-58
End-to-end delay








total delay from source to destination
N-1 routers on the path between source host and destination
host
assume no queueing
assume dproc is processing delay at each router (and source host)
transmission rate out of each router and out of source host is R
=> dtrans = L/R (L is size of packet)
propagation delay dprop of each link
What is the end-to-end delay?
dend-to-end = N ( dproc + dtrans + dprop )
Introduction 1-59
subject of intense research => more
about this in a couple of weeks




R: link bandwidth (bits/sec)
L: packet length (bits)
a: average packet arrival rate (in
packets/sec)
λ=a x L is arrival rate (bits/sec)



average queueing
delay
Queueing delay (revisited)
traffic intensity
= λ/R
λ/R ~ 0: avg. queueing delay small
λ/R 1: avg. queueing delay large
λ/R > 1: more “work” arriving
than can be serviced, average delay infinite!
http://wps.pearsoned.com/ecs_kurose_compnetw_6/216/55463/14198700.cw/index.html
La/R ~ 0
La/R
Introduction 1-60
1
Packet loss
queue that precedes the link in buffer has finite capacity
 packet arriving to full queue dropped (=> lost)
 lost packet may be retransmitted by previous node, by
source end system, or not at all

buffer
(waiting area)
A
packet being transmitted
B
packet arriving to
full buffer is lost
* Check out the Java applet for an interactive animation on queuing and loss
Introduction 1-61
“Real” Internet delays and routes
what do “real” Internet delay & loss look like?
 traceroute program: provides delay
measurement from source to router along endend Internet path towards destination. For all i:

 sends three packets that will reach router i on path
towards destination (=> 3N packets in total)
 router i will return packets to sender
 time interval between start of transmission and reply
(response time)
3 probes
3 probes
3 probes
Introduction 1-62
“Real” Internet delays, 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
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 trans-oceanic
8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms
link
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 * * *
* means no response (probe lost, router not replying)
18 * * *
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 1-63
Throughput

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
e.g. file of F bits is transferred within T seconds
=> average throughput?
F/T
server has
file of F bits
to send to client
link bandwidth
Rs bits/sec
link bandwidth
Rc bits/sec
Bits can be seen as
fluid that is pumped into pipe, bandwidth corresponds to link capacity!
Introduction 1-64
Throughput (more)

Rs < Rc What is average (approx) end-end
throughput?
Rs bits/sec

Rc bits/sec
Rs > Rc What is average (approx) end-end throughput?
Rs bits/sec
Rc bits/sec
File of size F needs F/min{Rc, Rs} time to be transmitted,
since at both links we cannot transmit faster than at
rate min{Rc, Rs}
(think about the last bit of the file!)
Introduction 1-65
Throughput (more)

R1,..., RN What is average end-end throughput?
min{R1,..., RN }
bottleneck link
link on end-end path that constrains end-end throughput
Introduction 1-66
Throughput: Internet scenario
10 simultaneous
downloads involving 10
client-server pairs
 all through a link with
rate R
 per-connection end-end
throughput?

min{Rc,Rs,R/10}
=> link may be a
bottleneck
 in practice: Rc or Rs is
often bottleneck
(because link in the core
has very high rate)

...
Rs
Rs
Rs
R
Rc
Rc
Rc
...
10 connections (fairly) share
backbone bottleneck link R bits/sec
Introduction 1-67
Bits and Bytes, Kilo, Mega, Giga
In concrete examples, we might have to convert between
bits and bytes, kilo-x, mega-x, giga-x, tera-x
1 bit is either a 1 or 0 (b)
upper case/lower case
8 bits = 1 byte (B)
1024 bytes = 1 Kibibyte (KiB) = 2^10 bytes
1024 Kibibytes = 1 Mebibyte (MiB) = 2^20 bytes
1024 Mebibytes = 1 Gibibyte (GiB) = 2^30 bytes
1024 Gibibytes = 1 Tebibyte (TiB) = 2^40 bytes
Common prefixes:
- kilox (Kx) -> 1,000 =10^3 (one thousand)
- megax (Mx) -> 1,000,000=10^6 (one million)
- gigax (Gx) -> 1,000,000,000=10^9 (one billion)
- terax (Tx) ->1,000,000,000,000= 10^12 (one trillion)
Introduction 1-68
Bits and Bytes, Kilo, Mega, Giga
In concrete examples, we might have to convert between
bits and bytes, kilo-x, mega-x, giga-x, tera-x
10 Mb to KB = (10*10^6)/(8*10^3) KB = 1250 KB
1500 KB to Mb = (1500*10^3*8)/10^6 Mb = 12 Mb
1 GiB to MB = (2^30)/10^6 MB ≈ 1073.74 MB
1 TB to GiB = (10^12)/2^30 GiB ≈ 931.32 GiB
Introduction 1-69
Chapter 1: 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
Introduction 1-70
Protocol “layers”
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?
…. or at least our
discussion of networks?
Introduction 1-71
Organization of air travel
ticket (purchase)
ticket (complain
if trip was bad)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing

a series of steps/ actions
Introduction 1-72
Layering of airline functionality
ticket (purchase)
ticket (complain)
ticket
baggage (check)
baggage (claim
baggage
gates (load)
gates (unload)
gate
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
airport
layers: each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
Introduction 1-73
Why layering?
dealing with complex systems:
allows to identify the relationships of the complex
pieces of the system
 modularization eases maintenance, updating of
system

 e.g., change in gate procedure (say, order in which
passengers are boarding) doesn’t affect rest of system
Introduction 1-74
Network protocols

application: supporting network
applications
 FTP, SMTP, HTTP, DNS
application
transport
network
link
physical
Internet protocol stack
Introduction 1-75
Network protocols

application: supporting network
applications
 FTP, SMTP, HTTP, DNS

transport: process-process data
transfer
 TCP (guaranteed delivery,
congestion control),
 UDP (connectionless)
 transport-layer packets = segments
application
transport
network
link
physical
Internet protocol stack
Introduction 1-76
Network protocols

application: supporting network
applications
 FTP, SMTP, HTTP

transport: process-process data
transfer
 TCP, UDP
 transport-layer packets = segments

network: routing of datagrams
from source to destination
 IP Protocol, routing protocols
application
transport
network
link
physical
Internet protocol stack
Introduction 1-77
Network protocols

application: supporting network
applications
 FTP, SMTP, HTTP

transport: process-process data
transfer
 TCP, UDP
 transport-layer packets = segments

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”
application
transport
network
link
physical
Internet protocol stack
Introduction 1-78
Network protocols
packets are called ...
application
messages
transport
segments
network
datagrams
link
frames
physical
Internet protocol stack
Introduction 1-79
Encapsulation
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame
M
Hl Hn Ht
application
transport
network
link
physical
link
physical
Paket switches implement only the bottom layers – they are
‘transparent‘ to the hosts and routers!
Encapsulation (slightly) increases frame size.
M
Ht
M
Hn Ht
M
Hl Hn Ht
M
destination
Hn Ht
M
application
transport
network
link
physical
Hl Hn Ht
M
network
link
physical
switch
Hn Ht
M
router
extract enclosed
datagram
Introduction 1-80
Chapter 1: 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
Introduction 1-81
Network security

field of network security:
 how bad guys can attack computer networks
 how we can defend networks against attacks
 how to design architectures that are immune to
attacks
=> part of security course offered every two years
at UdS
Introduction 1-82
Bad guys: put malware into hosts via Internet


malware (software doing bad things) can get in host
from:

virus: corrupts or modifies files on target host; typically user
runs infected program; self-replicating infection but cannot
be spread without a human action

worm: exploits security vulnerabilities to spread passively
(travel without any human action); does not attach itself to
an existing program; takes advantage of system's transport
features, which is what allows it to travel unaided
spyware can record keystrokes, web sites visited,
upload info to collection site
Introduction 1-83
Bad guys: attack server, network infrastructure
Denial of Service (DoS): attackers make resources
(server, bandwidth) unavailable to legitimate users
-
-
vulnerability attack: send message to vulnerable
application => service stops or host crashes
bandwidth flooding: send so many (fake) packets
that target’s access link becomes clogged
(‘verstopft’)
connection flooding: establish large number of TCP
connections (=> Chapter 3)
Introduction 1-84
Bad guys: attack server, network infrastructure
Distributed Denial of Service (DDoS)
bandwidth flooding:
1. select target
2. break into hosts around
the network
3. send packets to target from
compromised hosts
target
make sure that aggregate traffic
rate is approx. equal to access rate
of server
Introduction 1-85
Bad guys can sniff packets
packet “sniffing”:
 broadcast media (shared ethernet, wireless)
 network interface reads/records all packets (e.g.,
including passwords!) passing by
C
A
src:B dest:A

payload
B
best defense is strong encryption => cryptography
lecture at UdS
Introduction 1-86
Bad guys can use fake addresses
IP spoofing: send packet with false source address
(e.g. C pretends to be B )
C
A
src:B dest:A
payload
B
flood victim with fake traffic; attacker does not care about
responses to the attack packets
 each spoofed packet appears to come from a different address
(=> filtering is difficult!)
 best defense is end-point authentication (determine identity
of
Introduction
sender with certainty)
1-87

Chapter 1: 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
Introduction 1-88
Internet history
1961-1972: Early packet-switching principles





early 1960s: telephone
network was dominant
communication network
(circuit switching!!!)
... paket switching was
invented (Paul Baran)
1961: Kleinrock - queueing
theory shows effectiveness
of packet-switching
1964: Baran - packetswitching in military nets
1969: ARPAnet - first
packet-switched computer
network at MIT

1972:
 NCP (Network Control
Protocol) first host-host
protocol; predecessor of
TCP/IP
 first e-mail program (1971)
 file transfer (1973)
Introduction 1-89
Internet history
1961-1972: Early packet-switching principles



1961: Kleinrock - queueing
theory shows effectiveness
of packet-switching
1964: Baran - packetswitching in military nets
1969: ARPAnet - first
packet-switched computer
network at MIT

1972:
 NCP (Network Control
Protocol) first host-host
protocol; predecessor of
TCP/IP
 first e-mail program (1971)
 file transfer (1973)
nodes and connections of
the ARPA-net in 1974
Introduction 1-90
Internet history
1972-1980: Growing number of networks

1970: ALOHAnet satellite network in Hawaii (wireless paketbased radio network)

1970-1977: growing number of other stand-alone packetswitching networks

1974: Cerf and Kahn invented architecture for interconnecting
networks => early TCP

1976: Ethernet for wire-based shared broadcast networks

1979: ARPAnet had 200 nodes
Introduction 1-91
Internet history
1980-1990: new protocols and growths of networks

1983: deployment of TCP/IP in ARPAnet (replacing NCP)

1982: smtp e-mail protocol defined

1983: DNS defined for name-to-IP-address translation

1985: ftp protocol defined

1988: TCP congestion control
Introduction 1-92
Internet history
1990, 2000’s: commercialization, the Web, new apps
 early
1990’s: ARPAnet
decommissioned (replaced by a
new network of the NSF)
 early 1990s: World Wide Web
started as a CERN-project
(Switzerland)
 purpose: share information
among researchers.
 HTML, HTTP
 Mosaic, later Netscape
 late 1990’s: commercialization
of the Web
 1993: CERN announced that
WWW is free to anyone
late 1990’s – 2000’s:
 more killer apps: instant
messaging, P2P file sharing
(Napster!!!)
 about 50 million hosts, 100
million users
 thousands of startups creating
Internet products and
services
 Internet stocks collapsed in
2000-2001
 some winners: Microsoft,
Cisco, Yahoo, e-Bay, Google,
Amazon
http://www.w3.org/History/19921103-hypertext/hypertext/WWW/TheProject.html
Introduction 1-93
Internet history
2005-present




~350 million hosts in 2005, ~750 million hosts in 2010
Aggressive deployment of broadband access (DSL, Cable)
Increasing availability of high-speed wireless access
Emergence of online social networks:
 Facebook: since October 2012 one billion users (~ number of
habitants in Europe and the US)


Service providers (Google, Microsoft) create their own
networks
 Bypass Internet, providing “instantaneous” access to
search, email, etc.
E-commerce, universities, enterprises running their services in
“cloud” (e.g. email and Web hosting)=> end users access
cloud-based applications through a web browser
Introduction 1-94
Introduction: summary
covered a “ton” of material!







Internet overview
what’s a protocol?
network edge, core, access
network
 packet-switching versus
circuit-switching
 Internet structure
performance: loss, delay,
throughput
layering
security
history
you now have:


context, overview, “feel”
of networking
more depth, detail to
follow!
Introduction 1-95