Transcript 4th Edition: Chapter 1

Table of Contents
 Chapter 1 Introduction
 Chapter 2 Application Layer
 Chapter 3 Transport Layer
 Chapter 4 Network Layer
 Chapter 5 Link Layer and LAN
 Chapter 6 Wireless and Mobile Network
 Chapter 7 Multimedia Network
 Chapter 8 Network Security
 Chapter 9 Network Management
Introduction
1-1
Computer Networks
网络应用
（Email，Web，P2P，多媒体等）
网
络
管
理
网络互联
（TCP/IP、IP路由等）
物理网络
网
络
安
全
Chapter 1 Introduction
 what’s the Internet?
 what’s a protocol?
 network edge; hosts, access net, physical
media
 network core: packet/circuit switching,
Internet structure
 performance: loss, delay, throughput
 security
 protocol layers, service models
Introduction
1-3
Chapter 2 Application Layer
 network application models
 client-server paradigm
 peer-to-peer paradigm
 learn about protocols by examining popular
application-level protocols




HTTP
FTP
SMTP / POP3 / IMAP
DNS
 programming network applications

socket API
Introduction
1-4
Chapter 3 Transport Layer
 understand principles behind transport
layer services:
multiplexing/demultiplexing
 reliable data transfer
 flow control
 congestion control
 learn about transport layer protocols in the
Internet:




UDP: connectionless transport
TCP: connection-oriented transport
TCP congestion control
Introduction
1-5
Chapter 4 Network Layer
 network layer service models
 Virtual circuit and datagram networks
 forwarding versus routing
 how a router works
 What’s inside a router
 routing (path selection)
Routing algorithms
 Routing in the Internet

 dealing with scale
 advanced topics: IPv6, mobility
Introduction
1-6
Chapter 5 Link Layer and LAN
 understand principles behind data link layer
services:
error detection, correction
 sharing a broadcast channel: multiple access
 link layer addressing
 reliable data transfer, flow control: done!

 instantiation and implementation of various
link layer technologies
Introduction
1-7
Chapter 6: Wireless and Mobile Networks
 # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers!
 computer nets: laptops, palmtops, PDAs, Internetenabled phone promise anytime untethered
Internet access
 two important (but different) challenges


wireless: communication over wireless link
mobility: handling the mobile user who changes point of
attachment to network
Introduction
1-8
Chapter 7 Multimedia Network
Principles
 classify multimedia applications
 identify network services applications need
 making the best of best effort service
Protocols and Architectures
 specific protocols for best-effort
 mechanisms for providing QoS
 architectures for QoS
Introduction
1-9
Chapter 8 Network Security
 understand principles of network security:
cryptography and its many uses beyond
“confidentiality”
 authentication
 message integrity

 security in practice:
 firewalls and intrusion detection systems
 security in application, transport, network, link
layers
Introduction
1-10
Chapter 9 Network Management
 introduction to network management
motivation
 major components
 Internet network management framework
 MIB: management information base
 SMI: data definition language
 SNMP: protocol for network management
 security and administration
 presentation services: ASN.1

Introduction
1-11
Chapter 1
Introduction
Computer Networking:
A Top Down Approach ,
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, April
2009.
Introduction
1-12
Chapter 1: Introduction
Our goal:
 get “feel” 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, access





net, physical media
network core: packet/circuit
switching, Internet structure
performance: loss, delay,
throughput
security
protocol layers, service models
history
Introduction
1-13
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
1-14
What’s the Internet: “nuts and bolts” view
 millions of connected
PC
server
computing devices:
hosts = end systems
 running network
apps
 communication links
wireless
laptop
cellular
handheld
fiber, copper,
radio, satellite
 transmission
rate = bandwidth
 routers: forward
packets (chunks of
data)
access
points
wired
links
router
Mobile network
Global ISP
Home network
Regional ISP

Institutional network
Introduction
1-15
“Cool” internet appliances
Web-enabled toaster +
weather forecaster
IP picture frame
http://www.ceiva.com/
World’s smallest web server
http://www-ccs.cs.umass.edu/~shri/iPic.html
Internet phones
Introduction
1-16
What’s the Internet: “nuts and bolts” view

protocols control sending,
receiving of msgs


Mobile network
e.g., TCP, IP, HTTP, Skype,
Ethernet
Internet: “network of
networks”


loosely hierarchical
public Internet versus
private intranet
Global ISP
Home network
Regional ISP
Institutional network
 Internet standards
 RFC: Request for comments
 IETF: Internet Engineering
Task Force
Introduction
1-17
What’s the Internet: a service view
 communication
infrastructure enables
distributed applications:
 Web, VoIP, email, games,
e-commerce, file sharing
 communication services
provided to apps:
 reliable data delivery
from source to
destination
 “best effort” (unreliable)
data delivery
Introduction
1-18
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
1-19
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://www.awl.com/kurose-ross
2:00
<file>
time
Q: Other human protocols?
Introduction
1-20
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
1-21
A closer look at network structure:
 network edge:
applications and
hosts
 access networks,
physical media:
wired, wireless
communication links
 network core:
 interconnected
routers
 network of networks
Introduction
1-22
The network edge:
 end systems (hosts):



run application programs
e.g. Web, email
at “edge of network”
peer-peer
 client/server model


client host requests, receives
service from always-on server
client/server
e.g. Web browser/server;
email client/server
 peer-peer model:


minimal (or no) use of
dedicated servers
e.g. Skype, BitTorrent
Introduction
1-23
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
1-24
Dial-up Modem
central
office
home
PC



home
dial-up
modem
telephone
network
Internet
ISP
modem
(e.g., AOL)
Uses existing telephony infrastructure
 Home is connected to central office
up to 56Kbps direct access to router (often less)
Can’t surf and phone at same time: not “always on”
Digital Subscriber Line (DSL)
Existing phone line:
0-4KHz phone; 4-50KHz
upstream data; 50KHz-1MHz
downstream data
home
phone
Internet
DSLAM
telephone
network
splitter
DSL
modem
home
PC
central
office
Also uses existing telephone infrastruture
 up to 1 Mbps upstream (today typically < 256 kbps)
 up to 8 Mbps downstream (today typically < 1 Mbps)
 dedicated physical line to telephone central office

Residential access: cable modems
 Does not use telephone infrastructure
 Instead uses cable TV infrastructure
 HFC: hybrid fiber coax
asymmetric: up to 30Mbps downstream, 2
Mbps upstream
 network of cable and fiber attaches homes to
ISP router
 homes share access to router
 unlike DSL, which has dedicated access

Introduction
1-27
Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
Introduction
1-28
Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
home
Introduction
1-29
Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
Introduction
1-30
Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
home
Introduction
1-31
Cable Network Architecture: Overview
FDM (more shortly):
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
Channels
cable headend
cable distribution
network
home
Introduction
1-32
Fiber to the Home
ONT
optical
fibers
Internet
OLT
central office
ONT
optical
fiber
optical
splitter
ONT
 Optical links from central office to the home
 Two competing optical technologies:
 Passive Optical network (PON)
 Active Optical Network (PAN)
 Much higher Internet rates; fiber also carries
television and phone services
Ethernet Internet access
100 Mbps
Institutional
router
Ethernet
switch
To Institution’s
ISP
100 Mbps
1 Gbps
100 Mbps
server
 Typically used in companies, universities, etc
 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet
 Today, end systems typically connect into Ethernet
switch
Wireless access networks
 shared
wireless access
network connects end system
to router

via base station aka “access
point”
 wireless LANs:
 802.11b/g (WiFi): 11 or 54 Mbps
 wider-area wireless access
 provided by telco operator
 ~1Mbps over cellular system
(EVDO, HSDPA)
 next up (?): WiMAX (10’s Mbps)
over wide area
router
base
station
mobile
hosts
Introduction
1-35
Home networks
Typical home network components:
 DSL or cable modem
 router/firewall/NAT
 Ethernet
 wireless access
point
to/from
cable
headend
cable
modem
router/
firewall
Ethernet
wireless
laptops
wireless
access
point
Introduction
1-36
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
Twisted Pair (TP)
 two insulated copper
wires


Category 3: traditional
phone wires, 10 Mbps
Ethernet
Category 5:
100Mbps Ethernet
 unguided media:
 signals propagate freely,
e.g., radio
Introduction
1-37
Physical Media: coax, fiber
Coaxial cable:
Fiber optic cable:
conductors
 bidirectional
 baseband:
pulses, each pulse a bit
 high-speed operation:
 two concentric copper


single channel on cable
legacy Ethernet
 broadband:
 multiple channels on
cable
 HFC
 glass fiber carrying light

high-speed point-to-point
transmission (e.g., 10’s100’s Gps)
 low error rate: repeaters
spaced far apart ; immune
to electromagnetic noise
Introduction
1-38
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., Wifi)
 11Mbps, 54 Mbps
 wide-area (e.g., cellular)
 3G cellular: ~ 1 Mbps
 satellite
 Kbps to 45Mbps channel (or
multiple smaller channels)
 270 msec end-end delay
 geosynchronous versus low
altitude
Introduction
1-39
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
1-40
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
1-41
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
1-42
Network Core: Circuit Switching
network resources
(e.g., bandwidth)
divided into “pieces”
 pieces allocated to calls
 resource piece
idle if
 dividing link bandwidth
into “pieces”
 frequency division
 time division
not used by owning call
(no sharing)
Introduction
1-43
Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time
Introduction
1-44
T1标准
E1标准
Numerical example
 How long does it take to send a file of
640,000 bits from host A to host B over a
circuit-switched network?
All links are 1.536 Mbps
 Each link uses TDM with 24 slots/sec
 500 msec to establish end-to-end circuit

Let’s work it out!
Introduction
1-47
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
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
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

Node receives complete
packet before forwarding
Introduction
1-48
Packet Switching: Statistical Multiplexing
100 Mb/s
Ethernet
A
B
statistical multiplexing
C
1.5 Mb/s
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed pattern,
bandwidth shared on demand  statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
Introduction
1-49
Packet-switching: store-and-forward
L
R
R
 takes L/R seconds to
transmit (push out)
packet of L bits on to
link at R bps

store and forward:
entire packet must
arrive at router before
it can be transmitted
on next link
 delay = 3L/R (assuming
zero propagation delay)
R
Example:
 L = 7.5 Mbits
 R = 1.5 Mbps
 transmission delay = 15
sec
more on delay shortly …
Introduction
1-50
Circuit、Message and Packet Switching
Packet switching versus circuit switching
Packet switching allows more users to use network!
 1 Mb/s link
 each user:
 100 kb/s when “active”
 active 10% of time

circuit-switching:


10 users
N users
1 Mbps link
packet switching:

with 35 users,
probability > 10 active
at same time is less
than .0004
Q: how did we get value 0.0004?
Introduction
1-52
Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
 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 7)

Q: human analogies of reserved resources (circuit
switching) versus on-demand allocation (packet-switching)?
Introduction
1-53
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
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Introduction
1-54
Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone
peering
…
…
.
…
…
…
to/from customers
Introduction
1-55
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
1-56
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
1-57
Internet structure: network of networks
 a packet passes through many networks!
local
ISP
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
1-58
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
1-59
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
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Introduction
1-60
Four sources of packet delay
 1. nodal processing:
 check bit errors
 determine output link
 2. queueing
 time waiting at output
link for transmission
 depends on congestion
level of router
transmission
A
propagation
B
nodal
processing
queueing
Introduction
1-61
Delay in packet-switched networks
3. Transmission delay:
 R=link bandwidth (bps)
 L=packet length (bits)
 time to send bits into
link = L/R
transmission
A
4. Propagation delay:
 d = length of physical link
 s = propagation speed in
medium (~2x108 m/sec)
 propagation delay = d/s
Note: s and R are very
different quantities!
propagation
B
nodal
processing
queueing
Introduction
1-62
Caravan analogy
100 km
ten-car
caravan
toll
booth
 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?
100 km
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(2+60) minutes
Introduction
1-63
Caravan analogy (more)
100 km
ten-car
caravan
100 km
toll
booth
 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?
toll
booth
 Yes! After 7(1+6) 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
Introduction
1-64
Nodal delay
dnodal  dproc  dqueue  dtrans  dprop
 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
1-65
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!
http://wps.pearsoned.com/ecs_kurose_compnetw_6/216/55463/141987
Introduction
1-66
00.cw/index.html
“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
1-67
“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
1-68
Packet loss
 queue (aka buffer) preceding link in buffer has
finite capacity
 packet arriving to full queue dropped (aka lost)
 lost packet may be retransmitted by previous
node, by source end system, or not at all
buffer
(waiting area)
A
B
packet being transmitted
packet arriving to
full buffer is lost
Introduction
1-69
Throughput
 throughput: rate (bits/time unit) at which
bits transferred between sender/receiver
instantaneous: rate at given point in time
 average: rate over long(er) 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
1-70
Throughput (more)
 Rs
< Rc What is average end-end throughput?
Rs bits/sec
 Rs
Rc bits/sec
> 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
1-71
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
1-72
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
1-73
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
1-74
Organization of air travel
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
 A series of steps
Introduction
1-75
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
1-76
Layering of mail system
用 户
（写信人）
用户间约定
用 户
（收信人）
用户子系统
用户/邮局
约定
邮局间约定
邮
局
邮局/运输
部门约定
邮
局
运输部门间约定
运输部门
甲地
运输部门
乙地
邮局子系统
运输子系统
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
1-78
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
1-79
(5) Application Layer
RPC
111
SNMP
TFTP
SMTP
FTP
TELNET
161
69
25
21
23
UDP
TCP
IP
RPC
111
SNMP
TFTP
SMTP
FTP
161
69
25
21
UDP
TELNET
23
TCP
IP
Introduction
1-80
Application Layer
 Scope

Process-Process
 Data Unit
 Block/file/…
 Application Layer Protocol Application 
Software

SMTP Email  Outlook/Mail-Server
 http
 WWW IE/IIS Server
Introduction
1-81
Example: HTTP response message
1.status line
(protocol status code status phrase)
HTTP/1.1 200 OK
Connection close
Date: Thu, 06 Aug 1998 12:00:15 GMT
Server: Apache/1.3.0 (Unix)
2.header
Last-Modified: Mon, 22 Jun 1998 …...
lines
Content-Length: 6821
Content-Type: text/html
blank line
3.data, e.g.,
requested
HTML file
data data data data data ...
Introduction
1-82
(4) Transport Layer
Applications
Server 1
UDP
TCP
6
UDP
TCP
6
17
IP
Host D
Applications
Server 1
Transp
17
IP
Internet
Introduction
1-83
Transport Layer
 Scope
 End-to-end
 Data Unit

Segment (段)
 Function
Process Addressing
 Error Control
 Flow Control
 QoS Provision

 Examples
 TCP, UDP
Introduction
1-84
（3）Network Layer
Host A
P2
P1
P2
Packet 1
Packet 2
P2
Data
C
P2
Packet N
P1
P1
D
P1
P2
Data
P1
P1
P2
Host B
Introduction
1-85
Network Layer
 Scope
 Network Wide ,Between any two Hosts
 Data Unit
 Packet (分组)
 Function
 Inter-networking (网络互连)
 Routing (路由)
 Congestion Control (拥塞控制)
 Examples
 IP, IPX
Introduction
1-86
（2） Data Link Layer
 Scope
 Link ,Between Neighboring Points
 Data Unit
 Frame (帧)
 Function
 Error Control
 Flow Control
 2 Sub-layers
 MAC （Media Access Control，介质访问控制）
 LLC （Logical Link Control，逻辑链路控制）
 Examples
 SDLC, PPP, HDLC
Introduction
1-87
（1）Physical Layer
 Example

EIA RS-232-C， RS-449，RS-422，RS-423，RS-530

CCITT V.24，V.28，V.35，X.21，X.3，X.28，X.29

IEEE 10Base5，10Base2，10BaseT
Introduction
1-88
Physical Layer
 Mechanical Feature
1
2
3
4
5
6
7
8
9 10
14
15
16
17
18
19
20
21
22
11
23
12
24
13
25
RS-232-C 25 Pins
Introduction
1-89
Physical Layer
 Electrical Feature
MC1488
TTL
电平
“1”< -3V
“0”> +3V
MC1489
TTL
电平
Introduction
1-90
Physical Layer
 Signal Definition
DTE
计算机
或
终端
①
②
③
④
⑤
⑥
⑦
⑧
保护地(Protective Ground)
发送(TxD)
接收(RxD)
请求发送(RTS)
允许发送(CTS)
DCE就绪(DSR)
信号地(Signal Ground)
DCE
调制
解调器
载波检测(DCD)
20
DTE就绪(DTR)
22
振铃指示(RI)
Introduction
1-91
ISO/OSI reference model
 presentation: allow applications to
interpret meaning of data, e.g.,
encryption, compression, machinespecific 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
1-92
Encapsulation
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
1-93
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
1-94
Network Security
 今年5月16日，美出炉《 International Strategy for
Cyberspace》，第一次将陆、海、空、太空、网络五大
空间并列，强调美国(网络空间)国家、全球化、机构与
个体意识。特别提到对would-be攻击者，必要时采取
先发制人策略，并明确表示有权动用任何手段，包括
外交、信息、军事和经济手段
 今年7月19日，美国防部出台《DoS Strategy for
Operating in Cyberspace》
Network Security
 当今世界，网络信息安全形势极其严峻。各国十分重
视信息安全，如美国多年来一直将信息安全技术列为
国防基础技术，并作为制信息权的关键手段。

2009年6月25日，美军成立网络战司令部；

2010年7月19日，我军成立总参信息保障基地；

2011年6月30日，我军成立总参信息化部。
Network Security
 attacks on Internet infrastructure:
 infecting/attacking hosts: malware, spyware,
worms, unauthorized access (data stealing, user
accounts)
 denial of service: deny access to resources
(servers, link bandwidth)
 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
1-97
What can bad guys do: malware?
 Spyware:
 Worm:
 infection by downloading
 infection by passively
web page with spyware
receiving object that gets
itself executed
 records keystrokes, web
sites visited, upload info
 self- replicating: propagates
to collection site
to other hosts, users
 Virus
 infection by receiving
object (e.g., e-mail
attachment), actively
executing
 self-replicating:
propagate itself to
other hosts, users
Sapphire Worm: aggregate scans/sec
in first 5 minutes of outbreak (CAIDA, UWisc data)
Introduction
1-98
网络安全问题日益突出

攻击类型
•
•
•
•
•
信息窃取
电子欺骗
拒绝服务
病毒
非法入侵
 网络各层均存在攻击行为
• 终端, 局域网, 交换机, 服务器, 应用等
 攻击的复杂性增加
• 工具包
• 协作式攻击

无线 & 移动性
• 无线协议安全性还是不好
• 移动设备物理上就是不安全的
上网计算机上受到的攻击数据统计
浏览器配置被修改
71.9%
网络系统无法使用
50.1%
数据或文件被损坏
45.0%
操作系统崩溃
41.5%
32.3%
QQ、MSN密码，邮箱帐号被盗
网络游戏密码，虚拟资产被盗
18.5%
受到远程控制
10.8%
电脑使用变慢
7.9%
没有造成实质危害
3.1%
网上银行密码、帐号被盗
2.5%
网络系统运行慢
1.7%
其他
2%
0%
20%
40%
60%
80%
100%
Denial of Service attacks
 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 malware)
3. send packets toward
target from
compromised hosts
target
Introduction
1-101
真实案例：美国、韩国网站被攻击疑案
 2009年8月，韩国多个政府网站持续遭到不明黑客发动的分布
式拒绝服务攻击，造成部分网站瘫痪长达4个小时。
 韩国国家情报院院长元世勋声称，追踪到这些攻击来自于全球
16个国家和地区的86个IP地址。尽管韩方猜测攻击极有可能来
自于朝鲜，但苦于找不到足够的证据，因而不能确定黑客真实
身份，也不能还原出完整的攻击过程。
僵尸网络
 僵尸网络（Botnet）：使一组联网计算机感染僵尸(Bot) 病
毒，进而被控制用于发起各种网络攻击行为，从而导致整
个基础网络瘫痪或信息系统崩溃，亦可导致机密信息泄漏
或网络欺诈等犯罪活动。

拒绝服务攻击

发送垃圾邮件

窃取秘密

滥用资源

。。。
伪造的商业网站示例
中国银行网站
正确:
http://www.bank-of-china.com
假冒:
http://www.bank-off-china.com
农业银行
工商银行
www.95599.cn
www.icbc.com.cn
建设银行
www.ccb.cn
“钓鱼”邮件示例
Sniff, modify, delete your 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
Ethereal software used for end-of-chapter labs
is a (free) packet-sniffer
 more on modification, deletion later
Introduction

1-106
Packet sniffing:
Masquerade as you
 IP
spoofing: send packet with false source address
C
A
src:B dest:A
payload
B
Introduction
1-108
Masquerade as you
 IP
spoofing: send packet with false source address
 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
1-109
Masquerade as you
 IP
spoofing: send packet with false source address
 record-and-playback: sniff sensitive info (e.g.,
password), and use later
 password holder is that user from system point of
view
later …..
A
C
src:B dest:A
user: B; password: foo
B
Introduction
1-110
Honey Pot
Chicken
Hiker
Network Security
 more throughout this course
 Chapter 8: focus on security
 Crypographic techniques: obvious uses and
not so obvious uses
Introduction
1-112
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
1-113
Internet History
1961-1972: Early packet-switching principles
 1961: Kleinrock - queueing
theory shows
effectiveness of packetswitching
 1964: Baran - packetswitching 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
Introduction
1-114
Initial Expansion of the ARPANET
Dec. 1969
July 1970
Apr. 1972
Mar. 1971
Sept. 1972
Introduction
RFC 527: ARPAWOCKY; RFC 602: The Stockings Were Hung by the Chimney
with Care
1-115
115
Internet History
1972-1980: Internetworking, new and proprietary nets
 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
Introduction
1-116
Internet History
1980-1990: new protocols, a proliferation of networks
 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
Introduction
1-117
Internet History
1990, 2000’s: commercialization, the Web, new apps
 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
Growth of the Internet
in Terms of Number of Hosts
Number of Hosts on Web:
Aug. 1981
213
Oct. 1984
1,024
Dec. 1987
28,174
Oct. 1990
313,000
Jul. 1993
1,776,000
Jul. 1996 19,540,000
Jul. 1999 56,218,000
Jul. 2004 285,139,000
Jul. 2005 353,284,000
Jul. 2006 439,286,000
Jul. 2007 489,774,000
CAIDA router
level view
Introduction
1-119
119
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: 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!
Homework
 Review Questions: 18, 23
 Problems: 4, 5, 6, 9, 15, 18, 24
 Discussion: 4
 Extensive Reading
 L. Kleinrock. History of the Internet and its
Flexible Future. IEEE Wireless Communications,
15(1), Feb. 2008