Transcript Basic Concepts

计算机网络
胥正川（XU Zhengchuan）
管理学院 信息管理与信息系统系
办公室：思源楼708室，电话：25011237
Email: [email protected]
1
自编教材：
上—网络基础知识材料汇编
下– 移动通信技术材料汇编
Course evaluation
• Class Contribution 10%
• Case study 10% （3人组）
• Presentation 10%
• Final Exam 70%
2
An Introduction
to Networking
Chapter 1
Updated January 2009
XU Zhengchuan
Fudan University
Part I: Basic Networks
Concepts
Concepts we will see
throughout the book
Figure 1-1: Basic Networking Concepts
• What Is a Network?
– A network is a transmission system that connects two
or more applications running on different computers.
Network
5
Figure 1-1: Basic Networking Concepts
• Client/Server Applications
– Most Internet applications are client/server applications
– Clients receive service from servers
– The client is often a browser
Client
Program
Server
Program
Services
Client Computer
Server Computer
6
Part II: The Nine Elements
of a Network
Although the idea of “network”
is simple, you must understand the
nine elements found in most networks
Figure 1-3: Elements of a Network
Client Application
Server Application
Message (Frame)
Client
Computer
Mobile
Client
Access
Line
Switch
2
1.
Networks connect
applications on different computers.
Switch
Switch
1
Trunk
Networks connect computers: 3
2.Line
Clients (fixed and mobile) and
3. Servers
Server
Computer
Outside
World
Wireless
Access Point
Router
8
Figure 1-3: Elements of a Network
Client Application
Server Application
Message (Frame)
Client
Computer
Switch
4.
1
Computers (and
routers)
Trunk
usually communicate
Line
by sending messages
Mobile
called frames
Client
Wireless
Access Point
Server
Computer
Switch
3
Outside
World
Router
9
Figure 1-3: Elements of a Network
Client Application
Server Application
Client
Message (Frame)
Sw2 Sends
Sw1
Sends
Sends
Frame
Frame
Frame
To Sw3
to Sw2
to Sw1
Switch 2
Client
Computer
Switch 1
Trunk
Line
Mobile
5.
Client Switches Forward
Frames Sequentially
Wireless
Access Point
Server
Sw3 Sends
Computer
Frame
to
Server
Switch 3
Outside
World
Switch
4
Router
10
Figure 1-5: Ethernet Switch Operation
C3- is out Port 15
Switching Table
Port
Host
10
A1-44-D5-1F-AA-4C
2
13
B2-CD-13-5B-E4-65
15
15
C3-2D-55-3B-A9-4F
C3-2D-55-3B-A9-4F
16
D4-47-55-C4-B6-F9
D4-47-55-C4-B6-F9
Switch
3
Frame to C3…
Port 15
Frame to C3…
1
A1-44-D5-1F-AA-4C
B2-CD-13-5B-E4-65
A1- sends a frame to C3-
C3-2D-55-3B-A9-4F
Switch sends frame to C311
Figure 1-3: Elements of a Network
消息（帧）
Client Application
Server Application
Message (Frame)
Switch
2
Access
Line
Client
Computer 6.
Switch
Wireless Access
1
Points Connect
Trunk
Wireless Stations
Line
to Switches
Mobile
Client
Wireless
Access Point
Server
Computer
Switch
3
Outside
World
Switch
4
Router
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Figure 1-3: Elements of a Network
Client Application
Server Application
Message (Frame)
Client
Computer
Switch
1
Mobile
Client
Switch
2
Access
Line
Trunk
Line
Server
7.
Routers connect networks Computer
to the outside world;
Switch
Treated just like computers
3
in single networks
Switch
Yes, single networks can
4
Wireless
contain
routers
Access Point
Outside
World
Router
13
Figure 1-3: Elements of a Network
Client Application
Access
Line
Server Application
8. Access Lines
Message (Frame)
Connect Computers
to Switches （接入线）
Switch
2
Client
Computer
Switch
1
Server
Computer
Switch
3
Trunk
Line
Mobile
Client
9. Trunk Lines Connect
Wireless
Switches to Switches
and
Access Point
Switches to Routers（中继线）
Outside
World
Switch
4
Router
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Figure 1-4: Packet Switching and Multiplexing
（多路复用）
Breaking Communications into
Small Messages is Called
Packet Switching, even if the
Messages are Frames
AC
Client
Computer A
AC
AC
AC
BD
AC
Trunk Line
Access
Line Multiplexed Packets
BD
Share Trunk Lines
Mobile Client
Computer B
AC
Server
Computer C
BD
So Packet Switching
Reduces the Cost of Trunk Lines
BD
Router D
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Network Elements: Recap
• Name the 9 Elements of Single networks.
– Without looking back through
your handout
Never talk about an
innovation “reducing cost,”
“increasing speed,” etc.
without specifying
which element is
cheaper or faster.
For example, multiplexing
only reduces the cost of
trunk lines; other
costs are not decreased
16
Part III: Transmission
Speed
Figure 1-6: Transmission Speed
• Measuring Transmission Speed
– Measured in bits per second (bps)
– In metric notation:
• Increasing factors of 1,000 …
– Not factors of 1,024
• Kilobits per second (kbps)-note the lowercase k
• Megabits per second (Mbps)
• Gigabits per second (Gbps)
• Terabits per second (Tbps)
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Figure 1-6: Transmission Speed
• Measuring Transmission Speed
– What is 23,000 bps in metric notation?
– What is 3,000,000,000 in metric notation?
– What is 15,100,000 bps in metric notation?
• Occasionally measured in bytes per second
• If so, written as Bps
• Usually seen in file download speeds
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Figure 1-6: Transmission Speed
• Writing Transmission Speeds in Proper Form
– The rule for writing speeds (and metric numbers in
general) in proper form is that there should be 1 to 3
places before the decimal point
– 23.72 Mbps is correct (2 places before the decimal point).
– 2,300 Mbps has four places before the decimal point, so
it should be rewritten as 2.3 Gbps (1 place).
– 0.5 Mbps has zero places to the left of the decimal point.
It should be written as 500 kbps (3 places).
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Figure 1-6: Transmission Speed
• Writing Transmission Speeds in Proper Form
– How to convert 1,200 Mbps to proper form
• Divide the number 1,200 by 1000
– Move decimal point three places to the left: 1.200
• Multiply the metric suffix Mbps by 1,000
– Gbps
• Result:
– 1.2 Gbps
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Figure 1-6: Transmission Speed
• Writing Transmission Speeds in Proper Form
– How to convert 0.036 Mbps to proper form
• Multiply the number 0.036 by 1000
– Move decimal point three places to the right: 36
• Divide the metric suffix Mbps by 1,000
– kbps
• Result:
– 36 kbps
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Figure 1-6: Transmission Speed
• Writing Transmission Speeds in Proper Form
– How should you write the following in proper form?
• 549.73 kbps
• 0.47 Gbps
• 11,200 Mbps
• .0021 Gbps
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Figure 1-6: Transmission Speed
• Rated Speed（额定速率）
– The speed in bits per second that you should get
(advertised or specified in the standard).
• Throughput（吞吐率）
– The speed you actually get
– Almost always lower than the rated speed
• On Shared Transmission Lines
– Aggregate throughput—total throughput for all users
– Individual throughput—what individual users get
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Part IV: LANs and WANs
Figure 1-8: LANs Versus WANs
Characteristics
LANs
Scope
For transmission within For transmission
a site. Campus,
between sites
building, and SOHO
(Small Office or Home
Office) LANs
Building
LAN
Wide Area
Network
WANs
Campus
LAN
Home
LAN
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Figure 1-8: LANs Versus WANs
Characteristics
LANs
WANs
Cost per bit Transmitted
Low
High
Typical Speed
Unshared 100 Mbps
to a gigabit per
second to each
desktop. Even faster
trunk line speeds.
Shared 128 kbps to
several megabits per
second trunk line
speeds
It’s simple economics. If the cost per unit is higher, the number
of units demanded will be lower.
Corporations cannot afford high-speed for most of their WAN
transmission
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Figure 1-8: LANs Versus WANs
Characteristics
Management
Choices
LANs
WANs
On own premises, so
firm builds and
manages its own LAN
or outsources the
Work
Must use a carrier with
rights of way for
transmission in public
Area. Carrier handles
most work but
Charges a high price.
Unlimited
Only those offered by
carrier
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Figure 1-9: Local Area Network (LAN) in a
Large Building
Client
Server
Wall Jack
Workgroup Switch 2
Workgroup Switch 1
Wall Jack
To
WAN
Router
Core Switch
Frames from the client to the server go through Workgroup Switch 2,
through the Core Switch, through Workgroup Switch 1, and then to the
server
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Part V: Internets
Figure 1-11: Internets
• Single LANs Versus Internets
– In single networks (LANs and WANs), all devices
connect to one another by switches—our focus so far.
– In contrast, an internet is a group of networks connected
by routers so that any application on any host on any
single network can communicate with any application on
any other host on any other network in the internet.
Application
Application
LAN
LAN
WAN
Router
Router
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Figure 1-11: Internets
Host
• Internet Components
– All computers in an internet are called hosts
– Clients as well as servers
PDA
(Host)
Client PC
(Host)
VoIP Phone
(Host)
Internet
Server
(Host)
Cat
(Ignores
Internet)
Cellphone
(Host)
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Figure 1-11: Internets
• Hosts Have Two Addresses
• IP Address
– This is the host’s official address on its internet
– 32 bits long
– Expressed for people in dotted decimal notation (e.g.,
128.171.17.13)
• Single-Network Addresses
– This is the host’s address on its single network
– Ethernet addresses, for instance, are 48 bits long
– Expressed in hexadecimal notation (e.g., AF-23-9BE8-67-47)
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Figure 1-11: Internets
• Networks are connected by devices called routers
– Switches provide connections within networks, while
routers provide connections between networks in an
internet.
• Frames and Packets
– In single networks, message are called frames
– In internets, messages are called packets
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36
Figure 1-11: Internets
Packet
Frame
• Packets are carried within frames
– One packet is transmitted from the source host to the
destination host across the internet
• Its IP destination address is that of the destination
host
LAN
LAN
WAN
Router
Router
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Figure 1-11: Internets
Packet
Frame
• Packets are carried within frames
– In each network, the packet is carried in (encapsulated in)
a frame
– If there are N networks between the source and
destination hosts, there will be one packet and N
networks between the source and destination hosts,
there will be one packet and N frames for a transmission
LAN
LAN
WAN
Router
Router
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Figure 1-12: Internet with Three Networks
Host A
Packet
Network X
Network Z
A packet goes all the
way across the internet; Route A-B
It’s path is its route
R1
Network Y
R2
Host B
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Figure 1-12: Internet with Three Networks
In Network X, the Packet is Placed in Frame X
Frame X
Packet
Details in
Network X
Switch
Host A
10.0.0.23
AB-23-D1-A8-34-DD
Data link
A-R1
Switch
X1
A route is a packet’s
path through the Mobile
internetClient
Host
Route A-B
Network X
Switch
X2
Switch
Server
A data Link is a
Host
frame’s path through
its single network
Router R1
D6-EE-92-5F-C1-56
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Figure 1-12: Internet with Three Networks
Details in
Network Y
To
Network X
Route
A-B
Router R1
Data Link
R1-R2
To
Network Z
Frame Y
Packet
Router R2
AF-3B-E7-39-12-B5
Network Y
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Figure 1-12: Internet with Three Networks
Network Z
Data Link
R2-B
Host B
www.pukanui.com
1.3.45.111
55-6B-CC-D4-A7-56
Switch
Z1
Switch
Router R2
Switch
Z2
Switch
Mobile Client Host
Details in
Network Z
Frame Z
Packet
Router
Mobile Client
Computer
42
Figure 1-12: Internet with Three Networks
• In this internet with three networks, in a
transmission,
– There is one packet
– There are three frames (one in each network)
• If a packet in an internet must pass through 10
networks,
– How many packets will be sent?
– How many frames must carry the packet?
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Figure 1-13: Converting IP Addresses into
Dotted Decimal Notation
IP Address (32 bits long)
10000000101010110001000100001101
Divided into 4 bytes. These
10000000 10101011 00010001 00001101
are segments.
Convert each byte to
decimal (result will be
between 0 and 255)*
Dotted decimal notation
(4 segments separated by
dots)
128
171
17
13
128.171.17.13
*The conversion process is described in the Hands On section
at the end of the chapter.
44
Figure 1-17: The Internet
1.
User PC
Host
Computer
Access
Line
1.
Webserver
Host
Computer
3.
Internet Backbone
(Multiple ISP Carriers)
Access
Line
Router
NAP
ISP
NAP
ISP
NAP
ISP
ISP
2.
User PC’s
Internet Service
Provider
4.
NAPs = Network Access Points
Connect ISPs
2.
Webserver’s
Internet Service
Provider
45
Figure 1-18: Subnets in an Internet
LAN 2
LAN 1
Router
R1
LAN Subnet
60.4.3.x
LAN Subnet
10.1.x.x
LAN Subnet
10.2.x.x
LAN Subnet
10.3.x.x
Router R2
LAN Subnet
60.4.15.x
WAN
Subnet
123.x.x.x
Router
R4
LAN Subnet
60.4.7.x
LAN Subnet
60.4.131.x
Router R3
Note: Subnets are single networks (collections of switches, transmission lines)
Often drawn as simple lines to focus on routers for internetworking
46
Figure 1-19: Terminology Differences for SingleNetwork and Internet Professionals
By Single-Network
Professionals
By Internet
Professionals
Single Networks Are
Called
Networks
Subnets
Internets Are Called
Internets
Networks
In this book, we will usually call internets “internets”
and subnets “single networks”
47
Figure 1-14: The Internet, internets,
Intranets, and Extranets
• Lower-case internet
– Any internet
• Upper-case Internet
– The global Internet
• Intranet
– An internet restricted to users within a single company
• Extranet
– A group of resources that can be accessed by authorized
people in a group of companies
48
Figure 1-20: IP Address Management
• Every Host Must Have a Unique IP address
– Server hosts are given static IP addresses (unchanging)
– Clients get dynamic (temporary) IP addresses that may
be different each time they use an internet
• Dynamic Host Configuration Protocol (DHCP)
(Figure 1-21)
– Clients get these dynamic IP addresses from Dynamic
Host Configuration Protocol (DHCP) servers (Figure 1-21)
49
Figure 1-21: Dynamic Host Configuration
Protocol (DHCP)
1. DHCP Request Message:
“My 48-bit Ethernet address is A3-4E-CD-59-28-7F”.
Please give me a 32-bit IP address.”
Client PC
A3-4E-CD-59-28-7F
2. Pool of
IP Addresses
DHCP
Server
3. DHCP Response Message:
“Computer at A3-4E-CD-59-28-7F,
your 32-bit IP address is 11010000101111101010101100000010”.
(Usually other configuration parameters as well.)
50
Figure 1-20: IP Address Management
• Domain Name System (DNS) (Figure 1-22)
– IP addresses are official addresses on the Internet and
other internets
– Hosts can also have host names (e.g., cnn.com)
• Not official—like nicknames
– If you only know the host name of a host that you want to
reach, your computer must learn its IP address
• DNS servers tell our computer the IP address of a
target host whose name you know. (Figure 1-22)
51
Figure 1-22: The Domain Name System
(DNS)
1.
Client Host
wishes to reach
Voyager.cba.hawaii.edu;
Needs to know
its IP Address
DNS Table
Host Name
IP Address
…
…
…
…
Voyager.cba.hawaii.edu 128.171.17.13
…
…
2. Sends DNS Request Message
“The host name is Voyager.cba.hawaii.edu”
Voyager.cba.hawaii.edu
128.171.17.13
Local
DNS
Host
52
Figure 1-22: The Domain Name System
(DNS)
DNS Table
3.
DNS Host
looks up the
target host’s
IP address
Host Name
IP Address
…
…
…
…
Voyager.cba.hawaii.edu 128.171.17.13
…
…
4. DNS Response Message
“The IP address is 128.171.17.13”
5.
Client sends packets to
128.171.17.13
DNS
Host
Voyager.cba.hawaii.edu
128.171.17.13
53
Figure 1-22: The Domain Name System
(DNS)
The local DNS host
sends back the response;
the user is unaware that
other DNS hosts were involved
Client Host
DNS Table
Host Name
IP Address
…
…
…
…
Voyager.cba.hawaii.edu 128.171.17.13
…
…
Local
DNS
Host
1. DNS Request Message
3. DNS Response Message
If local DNS host does not
have the target host’s IP address,
it contacts other DNS hosts
to get the IP address
2.
Request &
Response
Anther DNS Host
54
Part VI: Security
Figure 1-23: Firewall and Hardened Hosts
Allowed Legitimate
Packet
Border
Firewall
Attacker
The
Internet
Hardened
Server
Border firewall
should pass
legitimate packets
Legitimate
Packet
Hardened
Client PC
Legitimate
Host
Log File
Internal
Corporate
Network
56
Figure 1-23: Firewall and Hardened Hosts
Hardened
Server
Border firewall
should deny (drop)
and log
attack packets
Border
Firewall
Attack
Packet
Attacker
The
Internet
Hardened
Client PC
Denied
Attack
Packet
Log File
Legitimate
Host
Internal
Corporate
Network
57
Figure 1-23: Firewall and Hardened Hosts
Hardened
Server
Attack
Packet
Border
Firewall
Attack
Packet
Attacker
The
Internet
Hardened
Client PC
Attack
Packet
Hosts should
Denied be hardened
Attack
against attack packets
Packet that get through
Legitimate
Host
Internal
Corporate
Log File
Network
58
Figure 1-24: Cryptographic Protections
• Cryptography
– The use of mathematical operations to thwart attacks on
message dialogues between pairs of communicating
parties (people, programs, or devices)
• Initial Authentication
– Determine the other party’s identity to thwart impostors
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Figure 1-24: Cryptographic Protections
• Message-by-Message Protections
– Encryption to provide confidentiality so that an
eavesdropper cannot reach intercepted messages
– Electronic signatures provide message-by-message
authentication to prevent the insertion of messages by
an impostor after initial authentication
– Electronic signatures usually also provide message
integrity; this tells the receiver whether anyone has
changed the message en route
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Topics Covered
Network Elements: Recap
• Applications (the only element that users care about)
• Computers
– Clients
– Servers
• Switches and Routers
• Transmission Lines
– Trunk lines
– Access Lines
• Messages (Frames)
• Wireless Access Points
Never talk about an
innovation “reducing cost,”
“increasing speed,” etc.
without specifying
which element is
cheaper or faster.
For example, multiplexing
only reduces the cost of
trunk lines; other
costs are not decreased
62
Recap: LANs and WANs
• LANs transmit data within
corporate sites
• WANs transmit data
between corporate sites
• Each LAN or WAN is a
single network
• LAN costs are low and
speeds are high
WAN
• WAN costs are high
and speeds are lower
63
Recap: Internets
• Most firms have multiple LANs and WANs.
• They must create internets
– An internet is a collection of networks connected
by routers so that any application on any host on
any single network can communicate with any
application on any other host on any other network
in the internet.
Application
Application
LAN
LAN
WAN
Router
Router
64
Recap: Internets
• Elements of an Internet
– Computers connected to the internet are called
hosts
• Both servers and client PCs are hosts
– Routers connect the networks of the internet
together
• In contrast, switches forward frames within
individual networks
Router
LAN
Client PC Host
Router
WAN
LAN
Server Host
65
Recap: Internets
• Hosts Have Two Addresses
• IP Address
– This is the host’s official address on its internet
– 32 bits long
– Expressed for people in dotted decimal notation (e.g.,
128, 171, 17.13)
• Single Network Addresses
– This is the host’s address on its single network
– Ethernet addresses, for instance, are 48 bits long
– Expressed in hexadecimal notation, e.g., AF-23-9BE8-67-47
66
Recap: Internets
• Switches versus Routers
– Switches move frames through a single network (LAN
or WAN)
– Routers move packets through internets
• Messages
– Messages in single networks are called frames
– Messages in internets are called packets
– Packets are encapsulated within (carried inside)
frames
67
Recap: Security
• Security
– Firewalls
– Hardened Hosts
– Cryptographic security
for sensitive dialogues
• Initial authentication
• Encryption for
confidentiality
• Electronic signatures for
authentication and
message integrity
68
Quality of Service
• It is not enough that networks work
– They must work well
• Quality of Service (QoS) defines quantitative
measures of service quality
– Speed
速度
– Delay (Latency)
延迟
– Reliability
可靠性
• Security (not a QoS measure but crucial)
69
Figure 1.9: Quality of Service (QoS)
• Speed
– Bits per second (bps)
– Multiples of 1,000 (not 1,024)
– Kilobits per second (kbps)—Note the Lower-case “k”
– Megabits（兆位）per second (Mbps)
– Gigabits （千兆位）per second (Gbps)
– Terabits （兆兆位）per second (Tbps)
– Petabits（千兆兆位）per second (Pbps)
70
Figure 1.9: Quality of Service (QoS)
• Congestion and Latency（拥塞和延迟）
– Congestion because traffic chronically or
momentarily exceeds capacity
– Latency delay measured in milliseconds (ms)
– Especially bad for some services such as voice
communication or highly interactive applications
71
Figure 1.9: Quality of Service (QoS)
• Reliability 可靠性
– Availability 可用性
• Percent of time the network is available to
users for transmission and reception
• Want 24x7x365 availability
• Telephone network: Five 9s (99.999%)
– Error Rate 错误率
• Percent of lost or damaged messages or bits
72
Figure 1.9: Quality of Service (QoS)
• Service Level Agreements (SLAs)
– Quantitative guarantees for various service parameters
– Example: Better than 99% availability and a packet loss
error rate of less 0.5% measured over each day; latency
not exceeding 45 ms 99% of the time.
– Network provider pays performance penalties if
guarantees are not met
73