Bài giảng Mạng máy tính 1 - Chapter 1: Course details - Phạm Trần Vũ

ck: security
1.7 History
Introduction 1-52
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 1-53
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 1-54
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 1-55
Caravan analogy
 cars “propagate” at 
100 km/hr
 toll booth takes 12 sec to 
service 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 1-56
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 1-57
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
proptransqueueprocnodal ddddd 
Introduction 1-58
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 1-59
“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 1-60
“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 measurements from 
gaia.cs.umass.edu to cs-gw.cs.umass.edu 
* means no response (probe lost, router not replying)
trans-oceanic
link
Introduction 1-61
Packet loss
 queue (aka buffer) preceding link in buffer has 
finite capacity
 packet arriving to full queue dropped (aka lost)
 lost packet may be retransmitted by previous 
node, by source end system, or not at all
A
B
packet being transmitted
packet arriving to
full buffer is lost
buffer 
(waiting area)
Introduction 1-62
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
server, with
file of F bits 
to send to client
link capacity
Rs bits/sec
link capacity
Rc bits/sec
pipe that can carry
fluid at rate
Rs bits/sec)
pipe that can carry
fluid at rate
Rc bits/sec)
server s nds bits 
(fluid) into pipe
Introduction 1-63
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
link on end-end path that constrains end-end throughput
bottleneck link
Introduction 1-64
Throughput: Internet scenario
10 connections (fairly) share 
backbone bottleneck link R bits/sec
Rs
Rs
Rs
Rc
Rc
Rc
R
 per-connection 
end-end 
throughput: 
min(Rc,Rs,R/10)
 in practice: Rc or 
Rs is often 
bottleneck
Introduction 1-65
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge
 end systems, access networks, links
1.3 Network core
 circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched 
networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction 1-66
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 1-67
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 1-68
ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departure
airport
arrival
airport
intermediate air-traffic
control centers
airplane routing airplane routing
ticket (complain)
baggage (claim
gates (unload)
runway (land)
airplane routing
ticket
baggage
gate
takeoff/landing
airplane routing
Layering of airline functionality
Layers: each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
Introduction 1-69
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 1-70
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 
neighboring network elements
 PPP, Ethernet
 physical: bits “on the wire”
application
transport
network
link
physical
Introduction 1-71
ISO/OSI reference model
 presentation: allow applications to 
interpret meaning of data, e.g., 
encryption, compression, machine-
specific conventions
 session: synchronization, 
checkpointing, recovery of data 
exchange
 Internet stack “missing” these 
layers!
 these services, if needed, must 
be implemented in application
 needed?
application
presentation
session
transport
network
link
physical
Introduction 1-72
source
application
transport
network
link
physical
HtHn M
segment Ht
datagram
destination
application
transport
network
link
physical
HtHnHl M
HtHn M
Ht M
M
network
link
physical
link
physical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulation
message M
M
frame
Introduction 1-73
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge
 end systems, access networks, links
1.3 Network core
 circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched 
networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction 1-74
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 1-75
Bad guys can put malware into 
hosts via Internet
 Malware can get in host from a virus, worm, or 
trojan horse.
 Spyware malware can record keystrokes, web 
sites visited, upload info to collection site.
 Infected host can be enrolled in a botnet, used 
for spam and DDoS attacks.
 Malware is often self-replicating: from an 
infected host, seeks entry into other hosts
Introduction 1-76
Bad guys can put malware into 
hosts via Internet
 Trojan horse
 Hidden part of some 
otherwise useful 
software
 Today often on a Web 
page (Active-X, plugin)
 Virus
 infection by receiving 
object (e.g., e-mail 
attachment), actively 
executing
 self-replicating: 
propagate itself to 
other hosts, users
 Worm:
 infection by passively 
receiving object that gets 
itself executed
 self- replicating: propagates 
to other hosts, users
Sapphire Worm: aggregate scans/sec
in first 5 minutes of outbreak (CAIDA, UWisc data)
Introduction 1-77
Bad guys can attack servers and 
network infrastructure
 Denial of service (DoS): attackers make resources 
(server, bandwidth) unavailable to legitimate traffic 
by overwhelming resource with bogus traffic
1. select target
2. break into hosts 
around the network 
(see botnet)
3. send packets toward 
target from 
compromised hosts
target
Introduction 1-78
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
A
B
C
src:B dest:A payload
 Wireshark software used for end-of-chapter 
labs is a (free) packet-sniffer
Introduction 1-79
The bad guys can use false source 
addresses
 IP spoofing: send packet with false source address
A
B
C
src:B dest:A payload
Introduction 1-80
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
B
C
src:B dest:A user: B; password: foo
Introduction 1-81
Network Security
more throughout this course
 chapter 8: focus on security
 crypographic techniques: obvious uses and 
not so obvious uses
Introduction 1-82
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge
 end systems, access networks, links
1.3 Network core
 circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched 
networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction 1-83
Internet History
 1961: Kleinrock - queueing 
theory shows 
effectiveness of packet-
switching
 1964: Baran - packet-
switching in military nets
 1967: ARPAnet conceived 
by Advanced Research 
Projects Agency
 1969: first ARPAnet node 
operational
 1972:
 ARPAnet public demonstration
 NCP (Network Control Protocol) 
first host-host protocol 
 first e-mail program
 ARPAnet has 15 nodes
1961-1972: Early packet-switching principles
Introduction 1-84
Internet History
 1970: ALOHAnet satellite 
network in Hawaii
 1974: Cerf and Kahn -
architecture for 
interconnecting networks
 1976: Ethernet at Xerox 
PARC
 ate70’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 1-85
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 1-86
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, P2P file sharing
 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 1-87
Internet History
2007:
 ~500 million hosts
 Voice, Video over IP
 P2P applications: BitTorrent 
(file sharing) Skype (VoIP), 
PPLive (video)
 more applications: YouTube, 
gaming
 wireless, mobility
Introduction 1-88
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, service models
 security
 history
You now have:
 context, overview, 
“feel” of networking
 more depth, detail to 
follow!