Introduction 1-1
Computer Networks
Zili Zhang(张自力 )
Faculty of Computer and Information Science
Southwest China Normal University
Introduction 1-2
Important info,About this Unit
?Textbook,
?Jim Kurose,Keith Ross
Computer Networking,A Top Down
Approach Featuring the Internet,
2nd edition,Addison-Wesley,July 2002,
?Contact me at zhangzl@swnu.edu.cn
Introduction 1-3
Important info,About this Unit
?To pass this unit:
?Assignments & Labs (40%) + Final Exam
(60%) >= 60%
?Assignments & Labs Marks >=60%
?Final Exam marks >=60%
?All assignments and labs are compulsory
?Buy the lab session guide from Ms Hao
Introduction 1-4
Important info,About this Unit
?No Cheating!
?No Plagiarism!
Introduction 1-5
Unit Outline
?Weeks 1-3,Computer networks and the
Internet
?Weeks 4-5,The application layer
?Weeks 6-7,Transport layer
?Weeks 8-9,Network layer
?Week 10,Routing
?Weeks 11-12,Link layer
?Weeks 13-14,Local Area Networks
?Weeks 15-16,Network Security
Introduction 1-6
Unit Outline
?Week 17,Network Management
?Weeks 18-19,Review
Introduction 1-7
Chapter 1
Computer Networks
and the Internet
Computer Networking,
A Top Down Approach
Featuring the Internet,
2nd edition,
Jim Kurose,Keith Ross
Addison-Wesley,July
2002,
The PowerPoint Slides are based on the material
provided by
J.F Kurose and K.W,Ross.
Introduction 1-8
Chapter 1,Introduction
Our goal:
? get context,
overview,“feel” of
networking
? more depth,detail
later in course
? approach:
?descriptive
?use Internet as
example
Overview:
? what’s the Internet
? what’s a protocol?
? network edge
? network core
? access net,physical media
? Internet/ISP structure
? performance,loss,delay
? protocol layers,service models
? history
Introduction 1-9
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-10
What’s the Internet:,nuts and bolts” view
? millions of connected
computing devices,hosts,
end-systems
? PCs workstations,servers
? PDAs phones,toasters
running network apps
? communication links
? fiber,copper,radio,
satellite
? transmission rate =
bandwidth
? routers,forward packets
(chunks of data)
local ISP
company
network
regional ISP
router workstation
server mobile
Introduction 1-11
“Cool” internet appliances
World’s smallest web server
http://www-ccs.cs.umass.edu/~shri/iPic.html
IP picture frame
http://www.ceiva.com/
Web-enabled toaster+weather forecaster
Introduction 1-12
What’s the Internet:,nuts and bolts” view
? protocols control sending,
receiving of msgs
? e.g.,TCP,IP,HTTP,FTP,PPP
? Internet:,network of
networks”
? loosely hierarchical
? public Internet versus
private intranet
? Internet standards
? RFC,Request for comments
? IETF,Internet Engineering
Task Force
local ISP
company
network
regional ISP
router workstation
server mobile
Introduction 1-13
What’s the Internet,a service view
? communication
infrastructure enables
distributed applications:
? Web,email,games,e-
commerce,database.,
voting,file (MP3) sharing
? communication services
provided to apps:
? connectionless
? connection-oriented
? cyberspace [Gibson]:
“a consensual hallucination experienced daily by
billions of operators,in every nation,...."
Introduction 1-14
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 1-15
What’s a protocol?
a human protocol and a computer network protocol:
Q,Other human protocols?
Hi
Hi
Got the
time?
2:00
TCP connection
req
TCP connection
response
Get http://www.awl.com/kurose-ross
<file>
time
Introduction 1-16
A closer look at network structure:
?network edge:
applications and
hosts
?network core:
?routers
?network of
networks
?access networks,
physical media:
communication links
Introduction 1-17
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-18
The network edge:
?end systems (hosts):
? run application programs
? e.g,Web,email
? at,edge of network”
?client/server model
? client host requests,receives
service from always-on server
? e.g,Web browser/server;
email client/server
?peer-peer model:
? minimal (or no) use of
dedicated servers
? e.g,Gnutella,KaZaA
Introduction 1-19
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 connection-
oriented 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 1-20
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 1-21
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-22
The Network Core
? mesh of interconnected
routers
? the fundamental
question,how is data
transferred through net?
?circuit switching:
dedicated circuit per
call,telephone net
?packet-switching,data
sent thru net in
discrete,chunks”
Introduction 1-23
Network Core,Circuit Switching
End-end resources
reserved for,call”
? link bandwidth,switch
capacity
? dedicated resources,
no sharing
? circuit-like
(guaranteed)
performance
? call setup required
Introduction 1-24
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
(no sharing)
? dividing link bandwidth
into,pieces”
?frequency division
?time division
Introduction 1-25
Circuit Switching,TDMA and TDMA
FDMA
frequency
time
TDMA
frequency
time
4 users
Example:
Introduction 1-26
Network Core,Packet Switching
each end-end data stream
divided into packets
? user A,B packets share
network resources
? each packet uses full link
bandwidth
? resources used as needed
resource contention:
? aggregate resource
demand can exceed
amount available
? congestion,packets
queue,wait for link use
? store and forward,
packets move one hop
at a time
?transmit over link
?wait turn at next
link
Bandwidth division into,pieces”
Dedicated allocation
Resource reservation
Introduction 1-27
Packet Switching,Statistical Multiplexing
Sequence of A & B packets does not have fixed
pattern ?statistical multiplexing.
In TDM each host gets same slot in revolving TDM
frame.
A
B
C10 MbsEthernet
1.5 Mbs
D E
statistical multiplexing
queue of packets
waiting for output
link
Introduction 1-28
Packet switching versus circuit switching
? 1 Mbit link
? each user,
? 100 kbps when,active”
? active 10% of time
? circuit-switching,
? 10 users
? packet switching,
? with 35 users,
probability > 10 active
less than,0004
Packet switching allows more users to use network!
N users
1 Mbps link
Introduction 1-29
Packet switching versus circuit switching
? Great for bursty data
?resource sharing
?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)
Is packet switching a,slam dunk winner?”
Introduction 1-30
Packet-switching,store-and-forward
? Takes L/R seconds to
transmit (push out)
packet of L bits on to
link or R bps
? Entire packet must
arrive at router before
it can be transmitted
on next link,store and
forward
? delay = 3L/R
Example:
? L = 7.5 Mbits
? R = 1.5 Mbps
? delay = 15 sec
R R R
L
Introduction 1-31
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
Introduction 1-32
Packet-switched networks,forwarding
? Goal,move packets through routers from source to
destination
? we’ll study several path selection (i.e,routing)algorithms
(chapter 4)
? datagram network:
? destination address in packet determines next hop
? routes may change during session
? analogy,driving,asking directions
? 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
? routers maintain per-call state
Introduction 1-33
Network Taxonomy
Telecommunication
networks
Circuit-switched
networks
FDM TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks
? Datagram network is not either connection-oriented
or connectionless.
? Internet provides both connection-oriented (TCP) and
connectionless services (UDP) to apps.
Introduction 1-34
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-35
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
Keep in mind,
? bandwidth (bits per
second) of access
network?
? shared or dedicated?
Introduction 1-36
Residential access,point to point access
? Dialup via modem
?up to 56Kbps direct access to
router (often less)
?Can’t surf and phone at same
time,can’t be,always on”
? ADSL,asymmetric digital subscriber line
?up to 1 Mbps upstream (today typically < 256 kbps)
?up to 8 Mbps downstream (today typically < 1 Mbps)
?FDM,50 kHz - 1 MHz for downstream
4 kHz - 50 kHz for upstream
0 kHz - 4 kHz for ordinary telephone
Introduction 1-37
Residential access,cable modems
? HFC,hybrid fiber coax
?asymmetric,up to 10Mbps upstream,1 Mbps
downstream
? network of cable and fiber attaches homes to
ISP router
?shared access to router among home
?issues,congestion,dimensioning
? deployment,available via cable companies,e.g.,
MediaOne
Introduction 1-38
Residential access,cable modems
Diagram,http://www.cabledatacomnews.com/cmic/diagram.html
Introduction 1-39
Cable Network Architecture,Overview
home
cable headend
cable distribution
network (simplified)
Typically 500 to 5,000 homes
Introduction 1-40
Cable Network Architecture,Overview
home
cable headend
cable distribution
network (simplified)
Introduction 1-41
Cable Network Architecture,Overview
home
cable headend
cable distribution
network
server(s)
Introduction 1-42
Cable Network Architecture,Overview
home
cable headend
cable distribution
network
Channels
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
D
A
T
A
D
A
T
A
C
O
N
T
R
O
L
1 2 3 4 5 6 7 8 9
FDM:
Introduction 1-43
Company access,local area networks
? company/univ local area
network (LAN) connects
end system to edge router
? Ethernet:
?shared or dedicated link
connects end system
and router
?10 Mbs,100Mbps,
Gigabit Ethernet
? deployment,institutions,
home LANs happening now
? LANs,chapter 5
Introduction 1-44
Wireless access networks
? shared wireless access
network connects end system
to router
? via base station aka,access
point”
? wireless LANs:
? 802.11b (WiFi),11 Mbps
? wider-area wireless access
? provided by telco operator
? 3G ~ 384 kbps
? Will it happen
? WAP/GPRS in Europe
base
station
mobile
hosts
router
Introduction 1-45
Home networks
Typical home network components,
? ADSL or cable modem
? router/firewall/NAT
? Ethernet
? wireless access
point
wireless
access
point
wireless
laptopsrouter/
firewall
cable
modem
to/from
cable
headend
Ethernet
(switched)
Introduction 1-46
Physical Media
? Bit,propagates between
transmitter/rcvr pairs
? physical link,what lies
between transmitter &
receiver
? guided media:
? signals propagate in solid
media,copper,fiber,coax
? unguided media:
? signals propagate freely,
e.g.,radio
Twisted Pair (TP)
? two insulated copper
wires
? Category 3,traditional
phone wires,10 Mbps
Ethernet
? Category 5 TP,
100Mbps Ethernet
Introduction 1-47
Physical Media,coax,fiber
Coaxial cable:
? two concentric copper
conductors
? bidirectional
? baseband:
? single channel on cable
? legacy Ethernet
? broadband:
? multiple channel on cable
? HFC
Fiber optic cable:
? glass fiber carrying light
pulses,each pulse a bit
? high-speed operation:
? high-speed point-to-point
transmission (e.g.,5 Gps)
? low error rate,repeaters
spaced far apart ; immune
to electromagnetic noise
Introduction 1-48
Physical media,radio
? signal carried in
electromagnetic
spectrum
? no physical,wire”
? bidirectional
? propagation
environment effects:
? reflection
? obstruction by objects
? interference
Radio link types:
? terrestrial microwave
? e.g,up to 45 Mbps channels
? LAN (e.g.,WaveLAN)
? 2Mbps,11Mbps
? wide-area (e.g.,cellular)
? e.g,3G,hundreds of kbps
? satellite
? up to 50Mbps channel (or
multiple smaller channels)
? 270 msec end-end delay
? geosynchronous versus
LEOS
Introduction 1-49
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-50
Internet structure,network of networks
? roughly hierarchical
? at center:,tier-1” ISPs (e.g.,UUNet,BBN/Genuity,
Sprint,AT&T),national/international coverage
?treat each other as equals
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1
providers
interconnect
(peer)
privately
NAP
Tier-1 providers
also interconnect
at public network
access points
(NAPs)
Introduction 1-51
Tier-1 ISP,e.g.,Sprint
Sprint US backbone network
Introduction 1-52
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 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
? tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP
Introduction 1-53
Internet structure,network of networks
?,Tier-3” ISPs and local ISPs
? last hop (“access”) network (closest to end systems)
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local
ISPlocalISP localISP
local
ISP
local
ISP Tier 3
ISP
local
ISP
local
ISP
local
ISP
Local and tier-
3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Introduction 1-54
Internet structure,network of networks
? a packet passes through many networks!
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local
ISPlocalISP localISP
local
ISP
local
ISP Tier 3
ISP
local
ISP
local
ISP
local
ISP
Introduction 1-55
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-56
How do loss and delay occur?
packets queue in router buffers
? packet arrival rate to link exceeds output link capacity
? packets queue,wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers,arriving packets
dropped (loss) if no free buffers
Introduction 1-57
Four sources of packet delay
? 1,nodal processing:
? check bit errors
? determine output link
A
B
propagation
transmission
nodal
processing queueing
? 2,queueing
? time waiting at output
link for transmission
? depends on congestion
level of router
Introduction 1-58
Delay in packet-switched networks
3,Transmission delay:
? R=link bandwidth (bps)
? L=packet length (bits)
? time to send bits into
link = L/R
4,Propagation delay:
? d = length of physical link
? s = propagation speed in
medium (~2x108 m/sec)
? propagation delay = d/s
A
B
propagation
transmission
nodal
processing queueing
Note,s and R are very
different quantities!
Introduction 1-59
Caravan analogy
? Cars,propagate” at
100 km/hr
? Toll booth takes 12 sec to
service a car
(transmission time)
? car~bit; caravan ~ packet
? Q,How long until caravan
is lined up before 2nd toll
booth?
? Time to,push” entire
caravan through toll
booth onto highway =
12*10 = 120 sec
? Time for last car to
propagate from 1st to
2nd toll both,
100km/(100km/hr)= 1 hr
? A,62 minutes
toll
booth
toll
booth
ten-car
caravan
100 km 100 km
Introduction 1-60
Caravan analogy (more)
? Cars now,propagate” at
1000 km/hr
? Toll booth now takes 1
min to service a car
? Q,Will cars arrive to
2nd booth before all
cars serviced at 1st
booth?
? Yes! After 7 min,1st car
at 2nd booth and 3 cars
still at 1st booth.
? 1st bit of packet can
arrive at 2nd router
before packet is fully
transmitted at 1st router!
? See Ethernet applet at AWL
Web site
toll
booth
toll
booth
ten-car
caravan
100 km 100 km
Introduction 1-61
Nodal delay
? 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
p r o pt r a n sq u e u ep r o cn o d a l ddddd ????
Introduction 1-62
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 1-63
“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 1-64
“Real” Internet delays and routes
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms
4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms
5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms
8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms
9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms
16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms
17 * * *
18 * * *
19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
traceroute,gaia.cs.umass.edu to www.eurecom.fr
Three delay measements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
* means no reponse (probe lost,router not replying)
trans-oceanic
link
Introduction 1-65
Packet loss
?queue (aka buffer) preceding link in buffer
has finite capacity
?when packet arrives to full queue,packet is
dropped (aka lost)
?lost packet may be retransmitted by
previous node,by source end system,or
not retransmitted at all
Introduction 1-66
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-67
Protocol,Layers”
Networks are complex!
? 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 1-68
Organization of air travel
?a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Introduction 1-69
Organization of air travel,a different view
Layers,each layer implements a service
?via its own internal-layer actions
?relying on services provided by layer below
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Introduction 1-70
Layered air travel,services
Counter-to-counter delivery of person+bags
baggage-claim-to-baggage-claim delivery
people transfer,loading gate to arrival gate
runway-to-runway delivery of plane
airplane routing from source to destination
Introduction 1-71
Distributed implementation of layer functionality
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
De
par
tin
g
air
po
rt
ar
rivi
ng
ai
rp
or
t
intermediate air traffic sites
airplane routing airplane routing
Introduction 1-72
Why layering?
Dealing with complex systems:
? explicit structure allows identification,
relationship of complex system’s pieces
?layered reference model for discussion
? modularization eases maintenance,updating of
system
?change of implementation of layer’s service
transparent to rest of system
?e.g.,change in gate procedure doesn’t affect
rest of system
? layering considered harmful?
Introduction 1-73
Internet protocol stack
? application,supporting network
applications
? FTP,SMTP,HTTP
? transport,host-host data transfer
? TCP,UDP
? network,routing of datagrams from
source to destination
? IP,routing protocols
? link,data transfer between
neighboring network elements
? PPP,Ethernet
? physical,bits,on the wire”
application
transport
network
link
physical
Introduction 1-74
Layering,logical communication
application
transport
network
link
physical
application
transport
network
link
physical application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
Each layer:
? distributed
?,entities”
implement
layer functions
at each node
? entities
perform
actions,
exchange
messages with
peers
Introduction 1-75
Layering,logical communication
application
transport
network
link
physical
application
transport
network
link
physical application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
data
data
E.g.,transport
? take data from app
? add addressing,
reliability check
info to form
“datagram”
? send datagram to
peer
? wait for peer to
ack receipt
? analogy,post
office
data
tra rt
tra ort
ack
Introduction 1-76
Layering,physical communication
application
transport
network
link
physical
application
transport
network
link
physical application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
data
data
Introduction 1-77
Protocol layering and data
Each layer takes data from above
? adds header information to create new data unit
? passes new data unit to layer below
application
transport
network
link
physical
application
transport
network
link
physical
source destination
M
M
M
M
Ht
HtHn
HtHnHl
M
M
M
M
Ht
HtHn
HtHnHl
message
segment
datagram
frame
Introduction 1-78
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 ISPs and Internet backbones
1.6 Delay & loss in packet-switched networks
1.7 Internet structure and ISPs
1.8 History
Introduction 1-79
Internet History
? 1961,Kleinrock - queueing
theory shows
effectiveness of packet-
switching
? 1964,Baran - packet-
switching in military nets
? 1967,ARPAnet conceived
by Advanced Research
Projects Agency
? 1969,first ARPAnet node
operational
? 1972:
? ARPAnet demonstrated
publicly
? NCP (Network Control
Protocol) first host-
host protocol
? first e-mail program
? ARPAnet has 15 nodes
1961-1972,Early packet-switching principles
Introduction 1-80
Internet History
? 1970,ALOHAnet satellite
network in Hawaii
? 1973,Metcalfe’s PhD thesis
proposes Ethernet
? 1974,Cerf and Kahn -
architecture for
interconnecting networks
? late70’s,proprietary
architectures,DECnet,SNA,
XNA
? late 70’s,switching fixed
length packets (ATM
precursor)
? 1979,ARPAnet has 200 nodes
Cerf and Kahn’s
internetworking principles:
? minimalism,autonomy -
no internal changes
required to
interconnect networks
? best effort service
model
? stateless routers
? decentralized control
define today’s Internet
architecture
1972-1980,Internetworking,new and proprietary nets
Introduction 1-81
Internet History
? 1983,deployment of
TCP/IP
? 1982,SMTP e-mail
protocol defined
? 1983,DNS defined
for name-to-IP-
address translation
? 1985,FTP protocol
defined
? 1988,TCP congestion
control
? new national networks,
Csnet,BITnet,
NSFnet,Minitel
? 100,000 hosts
connected to
confederation of
networks
1980-1990,new protocols,a proliferation of networks
Introduction 1-82
Internet History
? Early 1990’s,ARPAnet
decommissioned
? 1991,NSF lifts restrictions on
commercial use of NSFnet
(decommissioned,1995)
? early 1990s,Web
? hypertext [Bush 1945,Nelson
1960’s]
? HTML,HTTP,Berners-Lee
? 1994,Mosaic,later Netscape
? late 1990’s,
commercialization of the Web
Late 1990’s – 2000’s:
? more killer apps,instant
messaging,peer2peer
file sharing (e.g.,
Naptser)
? network security to
forefront
? est,50 million host,100
million+ users
? backbone links running
at Gbps
1990,2000’s,commercialization,the Web,new apps
Introduction 1-83
Introduction,Summary
Covered a,ton” of material!
? Internet overview
? what’s a protocol?
? network edge,core,access
network
?packet-switching versus
circuit-switching
? Internet/ISP structure
? performance,loss,delay
? layering and service
models
? history
You now have:
? context,overview,
“feel” of networking
? more depth,detail to
follow!
Computer Networks
Zili Zhang(张自力 )
Faculty of Computer and Information Science
Southwest China Normal University
Introduction 1-2
Important info,About this Unit
?Textbook,
?Jim Kurose,Keith Ross
Computer Networking,A Top Down
Approach Featuring the Internet,
2nd edition,Addison-Wesley,July 2002,
?Contact me at zhangzl@swnu.edu.cn
Introduction 1-3
Important info,About this Unit
?To pass this unit:
?Assignments & Labs (40%) + Final Exam
(60%) >= 60%
?Assignments & Labs Marks >=60%
?Final Exam marks >=60%
?All assignments and labs are compulsory
?Buy the lab session guide from Ms Hao
Introduction 1-4
Important info,About this Unit
?No Cheating!
?No Plagiarism!
Introduction 1-5
Unit Outline
?Weeks 1-3,Computer networks and the
Internet
?Weeks 4-5,The application layer
?Weeks 6-7,Transport layer
?Weeks 8-9,Network layer
?Week 10,Routing
?Weeks 11-12,Link layer
?Weeks 13-14,Local Area Networks
?Weeks 15-16,Network Security
Introduction 1-6
Unit Outline
?Week 17,Network Management
?Weeks 18-19,Review
Introduction 1-7
Chapter 1
Computer Networks
and the Internet
Computer Networking,
A Top Down Approach
Featuring the Internet,
2nd edition,
Jim Kurose,Keith Ross
Addison-Wesley,July
2002,
The PowerPoint Slides are based on the material
provided by
J.F Kurose and K.W,Ross.
Introduction 1-8
Chapter 1,Introduction
Our goal:
? get context,
overview,“feel” of
networking
? more depth,detail
later in course
? approach:
?descriptive
?use Internet as
example
Overview:
? what’s the Internet
? what’s a protocol?
? network edge
? network core
? access net,physical media
? Internet/ISP structure
? performance,loss,delay
? protocol layers,service models
? history
Introduction 1-9
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-10
What’s the Internet:,nuts and bolts” view
? millions of connected
computing devices,hosts,
end-systems
? PCs workstations,servers
? PDAs phones,toasters
running network apps
? communication links
? fiber,copper,radio,
satellite
? transmission rate =
bandwidth
? routers,forward packets
(chunks of data)
local ISP
company
network
regional ISP
router workstation
server mobile
Introduction 1-11
“Cool” internet appliances
World’s smallest web server
http://www-ccs.cs.umass.edu/~shri/iPic.html
IP picture frame
http://www.ceiva.com/
Web-enabled toaster+weather forecaster
Introduction 1-12
What’s the Internet:,nuts and bolts” view
? protocols control sending,
receiving of msgs
? e.g.,TCP,IP,HTTP,FTP,PPP
? Internet:,network of
networks”
? loosely hierarchical
? public Internet versus
private intranet
? Internet standards
? RFC,Request for comments
? IETF,Internet Engineering
Task Force
local ISP
company
network
regional ISP
router workstation
server mobile
Introduction 1-13
What’s the Internet,a service view
? communication
infrastructure enables
distributed applications:
? Web,email,games,e-
commerce,database.,
voting,file (MP3) sharing
? communication services
provided to apps:
? connectionless
? connection-oriented
? cyberspace [Gibson]:
“a consensual hallucination experienced daily by
billions of operators,in every nation,...."
Introduction 1-14
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 1-15
What’s a protocol?
a human protocol and a computer network protocol:
Q,Other human protocols?
Hi
Hi
Got the
time?
2:00
TCP connection
req
TCP connection
response
Get http://www.awl.com/kurose-ross
<file>
time
Introduction 1-16
A closer look at network structure:
?network edge:
applications and
hosts
?network core:
?routers
?network of
networks
?access networks,
physical media:
communication links
Introduction 1-17
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-18
The network edge:
?end systems (hosts):
? run application programs
? e.g,Web,email
? at,edge of network”
?client/server model
? client host requests,receives
service from always-on server
? e.g,Web browser/server;
email client/server
?peer-peer model:
? minimal (or no) use of
dedicated servers
? e.g,Gnutella,KaZaA
Introduction 1-19
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 connection-
oriented 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 1-20
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 1-21
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-22
The Network Core
? mesh of interconnected
routers
? the fundamental
question,how is data
transferred through net?
?circuit switching:
dedicated circuit per
call,telephone net
?packet-switching,data
sent thru net in
discrete,chunks”
Introduction 1-23
Network Core,Circuit Switching
End-end resources
reserved for,call”
? link bandwidth,switch
capacity
? dedicated resources,
no sharing
? circuit-like
(guaranteed)
performance
? call setup required
Introduction 1-24
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
(no sharing)
? dividing link bandwidth
into,pieces”
?frequency division
?time division
Introduction 1-25
Circuit Switching,TDMA and TDMA
FDMA
frequency
time
TDMA
frequency
time
4 users
Example:
Introduction 1-26
Network Core,Packet Switching
each end-end data stream
divided into packets
? user A,B packets share
network resources
? each packet uses full link
bandwidth
? resources used as needed
resource contention:
? aggregate resource
demand can exceed
amount available
? congestion,packets
queue,wait for link use
? store and forward,
packets move one hop
at a time
?transmit over link
?wait turn at next
link
Bandwidth division into,pieces”
Dedicated allocation
Resource reservation
Introduction 1-27
Packet Switching,Statistical Multiplexing
Sequence of A & B packets does not have fixed
pattern ?statistical multiplexing.
In TDM each host gets same slot in revolving TDM
frame.
A
B
C10 MbsEthernet
1.5 Mbs
D E
statistical multiplexing
queue of packets
waiting for output
link
Introduction 1-28
Packet switching versus circuit switching
? 1 Mbit link
? each user,
? 100 kbps when,active”
? active 10% of time
? circuit-switching,
? 10 users
? packet switching,
? with 35 users,
probability > 10 active
less than,0004
Packet switching allows more users to use network!
N users
1 Mbps link
Introduction 1-29
Packet switching versus circuit switching
? Great for bursty data
?resource sharing
?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)
Is packet switching a,slam dunk winner?”
Introduction 1-30
Packet-switching,store-and-forward
? Takes L/R seconds to
transmit (push out)
packet of L bits on to
link or R bps
? Entire packet must
arrive at router before
it can be transmitted
on next link,store and
forward
? delay = 3L/R
Example:
? L = 7.5 Mbits
? R = 1.5 Mbps
? delay = 15 sec
R R R
L
Introduction 1-31
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
Introduction 1-32
Packet-switched networks,forwarding
? Goal,move packets through routers from source to
destination
? we’ll study several path selection (i.e,routing)algorithms
(chapter 4)
? datagram network:
? destination address in packet determines next hop
? routes may change during session
? analogy,driving,asking directions
? 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
? routers maintain per-call state
Introduction 1-33
Network Taxonomy
Telecommunication
networks
Circuit-switched
networks
FDM TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks
? Datagram network is not either connection-oriented
or connectionless.
? Internet provides both connection-oriented (TCP) and
connectionless services (UDP) to apps.
Introduction 1-34
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-35
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
Keep in mind,
? bandwidth (bits per
second) of access
network?
? shared or dedicated?
Introduction 1-36
Residential access,point to point access
? Dialup via modem
?up to 56Kbps direct access to
router (often less)
?Can’t surf and phone at same
time,can’t be,always on”
? ADSL,asymmetric digital subscriber line
?up to 1 Mbps upstream (today typically < 256 kbps)
?up to 8 Mbps downstream (today typically < 1 Mbps)
?FDM,50 kHz - 1 MHz for downstream
4 kHz - 50 kHz for upstream
0 kHz - 4 kHz for ordinary telephone
Introduction 1-37
Residential access,cable modems
? HFC,hybrid fiber coax
?asymmetric,up to 10Mbps upstream,1 Mbps
downstream
? network of cable and fiber attaches homes to
ISP router
?shared access to router among home
?issues,congestion,dimensioning
? deployment,available via cable companies,e.g.,
MediaOne
Introduction 1-38
Residential access,cable modems
Diagram,http://www.cabledatacomnews.com/cmic/diagram.html
Introduction 1-39
Cable Network Architecture,Overview
home
cable headend
cable distribution
network (simplified)
Typically 500 to 5,000 homes
Introduction 1-40
Cable Network Architecture,Overview
home
cable headend
cable distribution
network (simplified)
Introduction 1-41
Cable Network Architecture,Overview
home
cable headend
cable distribution
network
server(s)
Introduction 1-42
Cable Network Architecture,Overview
home
cable headend
cable distribution
network
Channels
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
V
I
D
E
O
D
A
T
A
D
A
T
A
C
O
N
T
R
O
L
1 2 3 4 5 6 7 8 9
FDM:
Introduction 1-43
Company access,local area networks
? company/univ local area
network (LAN) connects
end system to edge router
? Ethernet:
?shared or dedicated link
connects end system
and router
?10 Mbs,100Mbps,
Gigabit Ethernet
? deployment,institutions,
home LANs happening now
? LANs,chapter 5
Introduction 1-44
Wireless access networks
? shared wireless access
network connects end system
to router
? via base station aka,access
point”
? wireless LANs:
? 802.11b (WiFi),11 Mbps
? wider-area wireless access
? provided by telco operator
? 3G ~ 384 kbps
? Will it happen
? WAP/GPRS in Europe
base
station
mobile
hosts
router
Introduction 1-45
Home networks
Typical home network components,
? ADSL or cable modem
? router/firewall/NAT
? Ethernet
? wireless access
point
wireless
access
point
wireless
laptopsrouter/
firewall
cable
modem
to/from
cable
headend
Ethernet
(switched)
Introduction 1-46
Physical Media
? Bit,propagates between
transmitter/rcvr pairs
? physical link,what lies
between transmitter &
receiver
? guided media:
? signals propagate in solid
media,copper,fiber,coax
? unguided media:
? signals propagate freely,
e.g.,radio
Twisted Pair (TP)
? two insulated copper
wires
? Category 3,traditional
phone wires,10 Mbps
Ethernet
? Category 5 TP,
100Mbps Ethernet
Introduction 1-47
Physical Media,coax,fiber
Coaxial cable:
? two concentric copper
conductors
? bidirectional
? baseband:
? single channel on cable
? legacy Ethernet
? broadband:
? multiple channel on cable
? HFC
Fiber optic cable:
? glass fiber carrying light
pulses,each pulse a bit
? high-speed operation:
? high-speed point-to-point
transmission (e.g.,5 Gps)
? low error rate,repeaters
spaced far apart ; immune
to electromagnetic noise
Introduction 1-48
Physical media,radio
? signal carried in
electromagnetic
spectrum
? no physical,wire”
? bidirectional
? propagation
environment effects:
? reflection
? obstruction by objects
? interference
Radio link types:
? terrestrial microwave
? e.g,up to 45 Mbps channels
? LAN (e.g.,WaveLAN)
? 2Mbps,11Mbps
? wide-area (e.g.,cellular)
? e.g,3G,hundreds of kbps
? satellite
? up to 50Mbps channel (or
multiple smaller channels)
? 270 msec end-end delay
? geosynchronous versus
LEOS
Introduction 1-49
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-50
Internet structure,network of networks
? roughly hierarchical
? at center:,tier-1” ISPs (e.g.,UUNet,BBN/Genuity,
Sprint,AT&T),national/international coverage
?treat each other as equals
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1
providers
interconnect
(peer)
privately
NAP
Tier-1 providers
also interconnect
at public network
access points
(NAPs)
Introduction 1-51
Tier-1 ISP,e.g.,Sprint
Sprint US backbone network
Introduction 1-52
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 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
? tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP
Introduction 1-53
Internet structure,network of networks
?,Tier-3” ISPs and local ISPs
? last hop (“access”) network (closest to end systems)
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local
ISPlocalISP localISP
local
ISP
local
ISP Tier 3
ISP
local
ISP
local
ISP
local
ISP
Local and tier-
3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Introduction 1-54
Internet structure,network of networks
? a packet passes through many networks!
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local
ISPlocalISP localISP
local
ISP
local
ISP Tier 3
ISP
local
ISP
local
ISP
local
ISP
Introduction 1-55
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-56
How do loss and delay occur?
packets queue in router buffers
? packet arrival rate to link exceeds output link capacity
? packets queue,wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers,arriving packets
dropped (loss) if no free buffers
Introduction 1-57
Four sources of packet delay
? 1,nodal processing:
? check bit errors
? determine output link
A
B
propagation
transmission
nodal
processing queueing
? 2,queueing
? time waiting at output
link for transmission
? depends on congestion
level of router
Introduction 1-58
Delay in packet-switched networks
3,Transmission delay:
? R=link bandwidth (bps)
? L=packet length (bits)
? time to send bits into
link = L/R
4,Propagation delay:
? d = length of physical link
? s = propagation speed in
medium (~2x108 m/sec)
? propagation delay = d/s
A
B
propagation
transmission
nodal
processing queueing
Note,s and R are very
different quantities!
Introduction 1-59
Caravan analogy
? Cars,propagate” at
100 km/hr
? Toll booth takes 12 sec to
service a car
(transmission time)
? car~bit; caravan ~ packet
? Q,How long until caravan
is lined up before 2nd toll
booth?
? Time to,push” entire
caravan through toll
booth onto highway =
12*10 = 120 sec
? Time for last car to
propagate from 1st to
2nd toll both,
100km/(100km/hr)= 1 hr
? A,62 minutes
toll
booth
toll
booth
ten-car
caravan
100 km 100 km
Introduction 1-60
Caravan analogy (more)
? Cars now,propagate” at
1000 km/hr
? Toll booth now takes 1
min to service a car
? Q,Will cars arrive to
2nd booth before all
cars serviced at 1st
booth?
? Yes! After 7 min,1st car
at 2nd booth and 3 cars
still at 1st booth.
? 1st bit of packet can
arrive at 2nd router
before packet is fully
transmitted at 1st router!
? See Ethernet applet at AWL
Web site
toll
booth
toll
booth
ten-car
caravan
100 km 100 km
Introduction 1-61
Nodal delay
? 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
p r o pt r a n sq u e u ep r o cn o d a l ddddd ????
Introduction 1-62
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 1-63
“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 1-64
“Real” Internet delays and routes
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms
4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms
5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms
8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms
9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms
16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms
17 * * *
18 * * *
19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
traceroute,gaia.cs.umass.edu to www.eurecom.fr
Three delay measements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
* means no reponse (probe lost,router not replying)
trans-oceanic
link
Introduction 1-65
Packet loss
?queue (aka buffer) preceding link in buffer
has finite capacity
?when packet arrives to full queue,packet is
dropped (aka lost)
?lost packet may be retransmitted by
previous node,by source end system,or
not retransmitted at all
Introduction 1-66
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched networks
1.7 Protocol layers,service models
1.8 History
Introduction 1-67
Protocol,Layers”
Networks are complex!
? 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 1-68
Organization of air travel
?a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Introduction 1-69
Organization of air travel,a different view
Layers,each layer implements a service
?via its own internal-layer actions
?relying on services provided by layer below
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Introduction 1-70
Layered air travel,services
Counter-to-counter delivery of person+bags
baggage-claim-to-baggage-claim delivery
people transfer,loading gate to arrival gate
runway-to-runway delivery of plane
airplane routing from source to destination
Introduction 1-71
Distributed implementation of layer functionality
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
De
par
tin
g
air
po
rt
ar
rivi
ng
ai
rp
or
t
intermediate air traffic sites
airplane routing airplane routing
Introduction 1-72
Why layering?
Dealing with complex systems:
? explicit structure allows identification,
relationship of complex system’s pieces
?layered reference model for discussion
? modularization eases maintenance,updating of
system
?change of implementation of layer’s service
transparent to rest of system
?e.g.,change in gate procedure doesn’t affect
rest of system
? layering considered harmful?
Introduction 1-73
Internet protocol stack
? application,supporting network
applications
? FTP,SMTP,HTTP
? transport,host-host data transfer
? TCP,UDP
? network,routing of datagrams from
source to destination
? IP,routing protocols
? link,data transfer between
neighboring network elements
? PPP,Ethernet
? physical,bits,on the wire”
application
transport
network
link
physical
Introduction 1-74
Layering,logical communication
application
transport
network
link
physical
application
transport
network
link
physical application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
Each layer:
? distributed
?,entities”
implement
layer functions
at each node
? entities
perform
actions,
exchange
messages with
peers
Introduction 1-75
Layering,logical communication
application
transport
network
link
physical
application
transport
network
link
physical application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
data
data
E.g.,transport
? take data from app
? add addressing,
reliability check
info to form
“datagram”
? send datagram to
peer
? wait for peer to
ack receipt
? analogy,post
office
data
tra rt
tra ort
ack
Introduction 1-76
Layering,physical communication
application
transport
network
link
physical
application
transport
network
link
physical application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
data
data
Introduction 1-77
Protocol layering and data
Each layer takes data from above
? adds header information to create new data unit
? passes new data unit to layer below
application
transport
network
link
physical
application
transport
network
link
physical
source destination
M
M
M
M
Ht
HtHn
HtHnHl
M
M
M
M
Ht
HtHn
HtHnHl
message
segment
datagram
frame
Introduction 1-78
Chapter 1,roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 ISPs and Internet backbones
1.6 Delay & loss in packet-switched networks
1.7 Internet structure and ISPs
1.8 History
Introduction 1-79
Internet History
? 1961,Kleinrock - queueing
theory shows
effectiveness of packet-
switching
? 1964,Baran - packet-
switching in military nets
? 1967,ARPAnet conceived
by Advanced Research
Projects Agency
? 1969,first ARPAnet node
operational
? 1972:
? ARPAnet demonstrated
publicly
? NCP (Network Control
Protocol) first host-
host protocol
? first e-mail program
? ARPAnet has 15 nodes
1961-1972,Early packet-switching principles
Introduction 1-80
Internet History
? 1970,ALOHAnet satellite
network in Hawaii
? 1973,Metcalfe’s PhD thesis
proposes Ethernet
? 1974,Cerf and Kahn -
architecture for
interconnecting networks
? late70’s,proprietary
architectures,DECnet,SNA,
XNA
? late 70’s,switching fixed
length packets (ATM
precursor)
? 1979,ARPAnet has 200 nodes
Cerf and Kahn’s
internetworking principles:
? minimalism,autonomy -
no internal changes
required to
interconnect networks
? best effort service
model
? stateless routers
? decentralized control
define today’s Internet
architecture
1972-1980,Internetworking,new and proprietary nets
Introduction 1-81
Internet History
? 1983,deployment of
TCP/IP
? 1982,SMTP e-mail
protocol defined
? 1983,DNS defined
for name-to-IP-
address translation
? 1985,FTP protocol
defined
? 1988,TCP congestion
control
? new national networks,
Csnet,BITnet,
NSFnet,Minitel
? 100,000 hosts
connected to
confederation of
networks
1980-1990,new protocols,a proliferation of networks
Introduction 1-82
Internet History
? Early 1990’s,ARPAnet
decommissioned
? 1991,NSF lifts restrictions on
commercial use of NSFnet
(decommissioned,1995)
? early 1990s,Web
? hypertext [Bush 1945,Nelson
1960’s]
? HTML,HTTP,Berners-Lee
? 1994,Mosaic,later Netscape
? late 1990’s,
commercialization of the Web
Late 1990’s – 2000’s:
? more killer apps,instant
messaging,peer2peer
file sharing (e.g.,
Naptser)
? network security to
forefront
? est,50 million host,100
million+ users
? backbone links running
at Gbps
1990,2000’s,commercialization,the Web,new apps
Introduction 1-83
Introduction,Summary
Covered a,ton” of material!
? Internet overview
? what’s a protocol?
? network edge,core,access
network
?packet-switching versus
circuit-switching
? Internet/ISP structure
? performance,loss,delay
? layering and service
models
? history
You now have:
? context,overview,
“feel” of networking
? more depth,detail to
follow!
