Chapter 1
Computer Networks
and the Internet
These slides are adapted from the slides that accompany the textbook
Textbook:
Computer Networking: A Top
Down Approach ,
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, April 2009.
http://www.aw-bc.com/kurose-ross
Introduction
1-1
Chapter 1: Introduction
Our goal:
Overview:
overview, “feel” of
networking
 more depth, detail
later in course
 approach:
 descriptive
 use Internet as
example
 what’s a protocol?
 get context,
 what’s the Internet
 network edge & access
 network core
 performance: loss, delay,
throughput, capacity
 protocol layers, service models
 security
Introduction
1-2
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Delay & loss in packet-switched networks
1.5 Protocol layers, service models
1.6 Security
1.7 History
Introduction
1-3
What’s the Internet: “nuts and bolts” view
 millions of connected
computing devices: hosts,
end-systems


PCs workstations, servers
PDAs phones, toasters
router
server

mobile
local ISP
running network apps
 communication links

workstation
regional ISP
fiber, copper, radio,
satellite
transmission rate =
bandwidth
 routers: forward packets
(chunks of data)
company
network
Introduction
1-4
What’s the Internet: “nuts and bolts” view
 protocols control sending,
receiving of msgs

e.g., TCP, IP, HTTP, FTP, PPP
 Internet standards
 RFC: Request for comments
 IETF: Internet Engineering
Task Force
router
server
workstation
mobile
local ISP
regional ISP
 Internet: “network of
networks”


loosely hierarchical
public Internet versus
private intranet
company
network
Introduction
1-5
Internet structure: network of networks
 roughly hierarchical
 at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T,
Cable and Wireless), national/international coverage
 treat each other as equals (peering points)
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Introduction
1-6
Internet structure: network of networks
 “Tier-2” ISPs: smaller (often regional) ISPs
 Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
 tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
Tier 1 ISP
Tier-2 ISPs
also peer
privately with
each other.
Tier-2 ISP
Tier-2 ISP
Introduction
1-7
Internet structure: network of networks
 “Tier-3” ISPs and local ISPs
 last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Introduction
1-8
Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone
peering
…
…
.
…
…
…
to/from customers
Introduction
1-9
POPs reside in buildings like this
London IXP
10
Internet Core Routers Look Like
These
Router on
“paper”
11
A closer look at network structure:
 network edge:
applications and
hosts
 network core:
 routers

network of
networks
 access networks,
physical media:
communication links
Introduction
1-12
What’s the Internet: a service view
 communication
infrastructure enables
distributed applications:

Web, email, games, ecommerce, database.,
voting, file (MP3) sharing
 communication services
provided to apps:


connectionless
connection-oriented
 services provided by
protocols

running on hosts and
routers.
Introduction
1-13
What’s a protocol?
human protocols:
 “what’s the time?”
 “I have a question”
 introductions
… specific msgs sent
… specific actions taken
when msgs received,
or other events
network protocols:
 machines rather than
humans
 all communication
activity in Internet
governed by protocols
protocols define format,
order of msgs sent and
received among network
entities, and actions
taken on msg
transmission, receipt
Introduction
1-14
What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/kurose-ross
2:00
<file>
time
Q: Other human protocols?
Introduction
1-15
Internet protocol stack
 application: supporting network
applications

FTP, SMTP, HTTP
 transport: process-process data
transfer

TCP, UDP
 network: routing of datagrams from
source to destination

IP, routing protocols
 link: data transfer between
application
transport
network
link
physical
neighboring network elements

PPP, Ethernet
 physical: bits “on the wire”
Introduction
1-16
Why stack or layering?
Dealing with complex systems (e.g., big c/c++
program)
 Need structured, modular design for easy maintenance




(layers = *.c files)
Layer n uses the services provided by layer n-1 (e.g.,
error/flow control, connection set up)
Interface between two layers, e.g., Application
Programmer’s Interface (API) or sockets
Logical connection between two layer n entities (peers)
layering harmful (can’t do global/customized
optimization) ?
2: Application Layer
17
encapsulation
at each layer
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
M
router
Introduction
1-18
An analogy (Fig.1-10 of Tanenbaum)
2: Application Layer
19
The Edge
 end systems (hosts):



run application programs
e.g. Web, email
at “edge of network”
 client/server model


peer-peer
client host requests, receives
service from always-on server
e.g. Web browser/server;
client/server
email client/server
 peer-peer model:


minimal (or no) use of
dedicated servers
e.g. Skype, BitTorrent
20
Network edge: connection-oriented service
Goal: data transfer
between end systems
 handshaking: setup
(prepare for) data
transfer ahead of time


Hello, hello back human
protocol
set up “state” in two
communicating hosts
 TCP - Transmission
Control Protocol

Internet’s connectionoriented service
TCP service [RFC 793]
 reliable, in-order byte-
stream data transfer

loss: acknowledgements
and retransmissions
 flow control:
 sender won’t overwhelm
receiver
 congestion control:
 senders “slow down sending
rate” when network
congested
Introduction
1-21
Network edge: connectionless service
Goal: data transfer
between end systems

same as before!
 UDP - User Datagram
Protocol [RFC 768]:
Internet’s
connectionless service
 unreliable data
transfer
 no flow control
 no congestion control
App’s using TCP:
 HTTP (Web), FTP (file
transfer), Telnet
(remote login), SMTP
(email)
App’s using UDP:
 streaming media,
teleconferencing, DNS,
Internet telephony
Introduction
1-22
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
2: Application Layer
23
Wireless access networks
 wireless LANs:
router
o 802.11b (2.4GHz): 11 Mbps;
o 802.11a (5GHz), 802.11g (2.4GHz): 54Mps access
point
o 802.11n (2.4 or 5GHz): up to 600Mps
(40Mhz channel and MIMO)
 Personal Area Networks (PANs)
o Bluetooth: upto 1 Mbps within ~ 10m
 Wide-area Cellular Access
o 2G (GSM, TDMA, CDMA, iDEN) 10-20kpbs (circuit-switched)
o 2.5G system (GPRS, 1xRTT, EDGE ) 30-50kbps ( up to 114kbps)
o 3G (UMTS, W-CDMA) 300 – 500kbps, (up to a few Mbps)
o 1xEvolutional Data Only (EV-DO) 2.4M/700K)
o 1XEV – DV (Data and Voice) (3.1M/1.5M) and HSPDA
o 4G: LTE (and LTE-Advanced) by 3GPP, TD-LTE in China, and
WiMax (by IEEE 802.16): about 100Mbps down and 50Mbps up.
2: Application Layer
24
Home networks
Typical home network components:
 cable (or DSL) modem
 router/firewall/NAT
 Ethernet
 wireless access point

DD-WRT, Tomato…
to/from
cable
headend
cable
modem
wireless
laptops
router/
firewall
Ethernet
(switched)
wireless
access
point
2: Application Layer
25
The Network Core
 mesh of interconnected
routers (wide area
networks or WANs)
 the fundamental
question: how is data
transferred through net?
 circuit switching (CS)
dedicated circuit per
call: e.g. telephone net
 packet-switching (PS)
data sent thru net in
discrete “chunks”
called “packets”
Introduction
1-26
Network Core: Circuit Switching
End-end resources
reserved for “call”
 call setup required
 link bandwidth, switch
capacity
 dedicated resources to
each call: no sharing
 multiple calls can be
multiplexed onto a link
 guaranteed performance
Introduction
1-27
Network Core: Circuit Switching
network resources
(e.g., bandwidth)
divided into “pieces”
 pieces allocated to calls
 resource piece idle if
not used by owning call
(dedicated, no sharing)
 not bandwidth efficient
if data is sporadic (or
bursty with a variable
bit rate)
 dividing link bandwidth into
“pieces” (for call grooming,
aggregating or multiplexing)
 frequency division
 (wavelength division)
• f x l = C (speed of light)
time division
 code division (e.g.,
wireless/cellular)
 space division (with a
multi-fiber link)

Introduction
1-28
Circuit Switching: TDMA and TDMA
Example:
FDMA
4 users
frequency
time
TDMA
frequency
time
Introduction
1-29
Electromagnetic Spectrum
Introduction
1-30
Network Core: Packet Switching
each end-end data stream
 each packet has a
divided into packets
“header” (containing
e.g., destination
 user A & B’s packets share
address) in addition to
network resources
“payload” (data)
 each packet uses full link
 Store and Forward
bandwidth
(requires buffer and
 resources used as needed
introduces delay)
 Statistical multiplexing
(support an aggregate
Bandwidth division into “pieces”
demand that exceeds
Dedicated allocation
link bandwidth)
Resource reservation
Introduction
1-31
Packet-switching: store-and-forward
L
R
 entire packet must
R
arrive at router before it
can be transmitted on
next link: store and
forward
 must have a large enough
buffer
 incurs transmission delay
per hop
 alternative: cut-through
switching as in CS
R
Example:
 transmit (push out) a
packet of L bits on to
link of R bps
 L = 7.5 Mbits
 R = 1.5 Mbps
 Transmission delay per
hop = L/R = 5 sec
 Total transmission
delay (3 hops) = 15 sec
Introduction
1-32
Four sources of delay (I)
1. Transmission delay:
 R=link bandwidth (bps)
 L=packet length (bits)
 time to send bits into
link = L/R
transmission
A
2. Propagation delay:
 d = length of physical link
 c = propagation speed in
medium (~2x108 m/sec)
 propagation delay = d/c
Note: c and R are very
different quantities!
propagation
B
nodal
processing
queueing
Introduction
1-33
Four sources of delay (II)
 3. nodal processing:


check bit errors
determine output link
(based on e.g., the
destination address)
 4. queueing
 time waiting at output link
for transmission (and at
input buffer too)
 depends on congestion
level of router
transmission
A
propagation
B
nodal
processing
queueing
Introduction
1-34
Nodal delay
d nodal = d proc  d queue  d trans  d prop
 dproc = processing delay
 typically a few microsecs or less
 dqueue = queuing delay
 depends on congestion
 dtrans = transmission delay
 = L/R, significant for low-speed links
 dprop = propagation delay
 a few microsecs to hundreds of msecs
Introduction
1-35
Queueing delay (revisited)
 R=link bandwidth (bps)
 L=packet length (bits)
 a=average packet
arrival rate
traffic intensity = La/R
 La/R ~ 0: average queueing delay small
 La/R -> 1: delays become large
 La/R > 1: more “work” arriving than can be
serviced, average delay infinite!
Introduction
1-36
“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
router i will return packets to sender
sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Introduction
1-37
“Real” Internet delays and routes
traceroute: gaia.cs.umass.edu to www.eurecom.fr
Three 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
Introduction
1-38
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

link
capacity
that
can carry
server,
with
server
sends
bits pipe
Rs bits/sec
fluid
at rate
file of
F bits
(fluid)
into
pipe
Rs bits/sec)
to send to client
link that
capacity
pipe
can carry
Rfluid
c bits/sec
at rate
Rc bits/sec)
Introduction
1-39
Throughput (more)
 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
1-40
Throughput: Internet scenario
 per-connection
end-end
throughput:
min(Rc,Rs,R/10)
 in practice: Rc or
Rs is often
bottleneck
Rs
Rs
Rs
R
Rc
Rc
Rc
10 connections (fairly) share
backbone bottleneck link R bits/sec
Introduction
1-41
Packet Switching: Statistical Multiplexing
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed pattern 
statistical (time division) multiplexing.
Unlike in TDM, where each host gets same slot in revolving
TDM frame.
Introduction
1-42
Packet switching versus circuit switching
Packet switching allows more users to use network!
 1 Mbit link
 each user:
 100 kbps when “active”
 active 10% of time
 circuit-switching:
 10 users
N users
1 Mbps link
 packet switching:
 with 35 users,
probability > 10 active
less than .0004
Introduction
1-43
Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
 Great for bursty data
resource sharing (statistical multiplexing)
 simpler, no call setup
 Excessive congestion: packet delay and loss
 protocols needed for reliable data transfer,
congestion control
 Q: How to provide circuit-like behavior?
 bandwidth guarantees needed for audio/video
apps
 still an unsolved problem (chapter 6)

Introduction
1-44
Message Switching (A mesg = 7.5Mbits)
at 1.5Mbps transmission rate
Introduction
1-45
Packet Switching: Message Segmenting
Now break up the message
into 5000 packets
 Each packet 1,500 bits
 1 msec to transmit
packet on one link
 pipelining: each link
works in parallel
 Delay reduced from 15
sec to 5.002 sec
 Q: the smaller the
packets, the better?
Introduction
1-46
Packet-switched networks: forwarding
 Goal: move packets through routers from source to destination

we’ll study several path selection (i.e. routing) algorithms (ch. 4)
 datagram network:



destination address in packet determines next hop (according to
the network’s routing algorithm)
routes may change during session (out of order delivery) due to
congested/failed links/routers
analogy: driving, asking directions at each/every intersection
 virtual circuit network:



each packet carries tag (virtual circuit ID), tag determines next
hop
fixed path determined at call setup time, remains fixed thru call
(can be manually chosen for traffic engineering purposes)
routers maintain per-call state
Introduction
1-47
VC vs Datagram
 With VC, two packets
with the same
destination can be
assigned two different
VC# and forced to
take different paths.
Introduction
1-48
Network Taxonomy
Telecommunication
networks
Circuit-switched
networks
FDM
TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks
• One may have a circuit-switched network (e.g. optical WDM)
below, over which a packet-switched network runs
• Internet provides both connection-oriented (TCP) and
connectionless services (UDP) to apps.
Introduction
1-49
Network Security
 The field of network security is about:
 how bad guys can attack computer networks
 how we can defend networks against attacks
 how to design architectures that are immune to
attacks
 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!

Introduction
1-50
The bad guys can sniff packets
Packet sniffing:
broadcast media (shared Ethernet, wireless)
 promiscuous network interface reads/records all
packets (e.g., including passwords!) passing by

C
A
src:B dest:A

payload
B
Wireshark software used for end-of-chapter
labs is a (free) packet-sniffer
Introduction
1-51
The bad guys can use false source
addresses
 IP spoofing: send packet with false source address
C
A
src:B dest:A
payload
B
Introduction
1-52
The bad guys can record and
playback
 record-and-playback: sniff sensitive info (e.g.,
password), and use later
 password holder is that user from system point of
view
A
C
src:B dest:A
user: B; password: foo
B
Introduction
1-53
Network Security
 more throughout this course
 A chapter focuses on security
 crypographic techniques: obvious uses and
not so obvious uses
Introduction
1-54