chương 2 (Mô hình OSI & Bộ giao thức TCP-IP) (1).pdf

10-Sep-23 1
CHƯƠNG 2
MÔ HÌNH OSI VÀ BỘ GIAO THỨC TCP/IP

TCP/IP Protocol Suite 2
OBJECTIVES:
❑ To discuss the idea of multiple layering in data communication
and networking and the interrelationship between layers.
❑ To discuss the OSI model and its layer architecture and to show
the interface between the layers.
❑ To briefly discuss the functions of each layer in the OSI model.
❑ To introduce the TCP/IP protocol suite and compare its layers
with the ones in the OSI model.
❑ To show the functionality of each layer in the TCP/IP protocol
with some examples.
❑ To discuss the addressing mechanism used in some layers of the
TCP/IP protocol suite for the delivery of a message from the
source to the destination.

2-1 PROTOCOL LAYERS
In Chapter 1, we discussed that a protocol is
required when two entities need to communicate.
When communication is not simple, we may
divide the complex task of communication into
several layers. In this case, we may need several
protocols, one for each layer.
Let us use a scenario in communication in
which the role of protocol layering may be better
understood. We use two examples. In the first
example, communication is so simple that it can
occur in only one layer.

Assume Maria and Ann are neighbors with a lot of common
ideas. However, Maria speaks only Spanish, and Ann speaks
only English. Since both have learned the sign language in their
childhood, they enjoy meeting in a cafe a couple of days per
week and exchange their ideas using signs. Occasionally, they
also use a bilingual dictionary. Communication is face to face
and Happens in one layer as shown in Figure 2.1.
Example 2.1

Figure 2.1 Example 2.1

Now assume that Ann has to move to another town
because of her job. Before she moves, the two meet for the
last time in the same cafe. Although both are sad, Maria
surprises Ann when she opens a packet that contains two
small machines. The first machine can scan and transform
a letter in English to a secret code or vice versa. The other
machine can scan and translate a letter in Spanish to the
same secret code or vice versa. Ann takes the first
machine; Maria keeps the second one. The two friends can
still communicate using the secret code, as shown in
Figure 2.2.
Example 2.2

Figure 2.2 Example 2.2

2-2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO) is a multinational body
dedicated to worldwide agreement on
international standards. Almost three-fourths of
countries in the world are represented in the ISO.
An ISO standard that covers all aspects of
network communications is the Open Systems
Interconnection (OSI) model. It was first
introduced in the late 1970s.

ISO is the organization;
OSI is the model.
Note

Figure 2.3 The OSI model

Figure 2.4 OSI layers

Figure 2.5 An exchange using the OSI model

The physical layer is responsible for
moving individual bits from one
(node) to the next.
Note

Figure 2.6 Summary of OSI Layers

Protocols supported at various levels
Layer 7 Application
SMTP, HTTP, FTP, POP3,
SNMP
Layer 6 Presentation MPEG, ASCH, SSL, TLS
Layer 5 Session NetBIOS, SAP
Layer 4 Transport TCP, UDP
Layer 3 Network
IPV5, IPV6, ICMP,
IPSEC, ARP, MPLS.
Layer 2 Data Link
RAPA, PPP, Frame
Relay, ATM, Fiber Cable,
etc.
Layer 1 Physical
RS232, 100BaseTX,
ISDN, 11.

Here, are major benefits/pros of using the OSI model:
• It helps you to standardize router, switch, motherboard, and other
hardware
• Reduces complexity and standardizes interfaces
• Facilitates modular engineering
• Helps you to ensure interoperable technology
• Helps you to accelerate the evolution
• Protocols can be replaced by new protocols when technology
changes.
• Provide support for connection-oriented services as well as
connectionless service.
• It is a standard model in computer networking.
• Supports connectionless and connection-oriented services.
• Offers flexibility to adapt to various types of protocols
Advantages of the OSI Model

Disadvantages of the OSI Model
Here are some cons/ drawbacks of using OSI Model:
• Fitting of protocols is a tedious task.
• You can only use it as a reference model.
• Doesn’t define any specific protocol.
• In the OSI network layer model, some services are duplicated in many
layers such as the transport and data link layers
• Layers can’t work in parallel as each layer need to wait to obtain data
from the previous layer.

2-3 TCP/IP PROTOCOL SUITE
The TCP/IP protocol suite was developed prior to
the OSI model. Therefore, the layers in the
TCP/IP protocol suite do not match exactly with
those in the OSI model. The original TCP/IP
protocol suite was defined as four software
layers built upon the hardware. Today, however,
TCP/IP is thought of as a five-layer model with
the layers named similarly to the ones in the OSI
model. Figure 2.7 shows both configurations.

Topics Discussed in the Section
✓Comparison between OSI and TCP/IP
✓Layers in the TCP/IP Suite

Figure 2.7 Layers in the TCP/IP Protocol Suite

Figure 2.8 TCP/IP and OSI model

Figure 2.9 A private internet

Figure 2.10 Communication at the physical layer
A
Physical
layer
Physical
layer
R1 R3 R4 B
Source Destination
Legend
011 ... 101
0
1
1
.
.
.
1
0
1
011 ... 101 011 ... 101
Link 3 Link 5 Link 6
Link 1

The unit of communication at the
physical layer is a bit.
Note

Figure 2.11 Communication at the data link layer
A
Physical Physical
Data link
Data link
R1 R3 R4 B
Source Destination Data
D Header
H
Legend
Link 1 Link 3 Link 5 Link 6
Frame
D2 H2
F
r
a
m
e
D
2
H
2
Frame
D2 H2
Frame
D2 H2

The unit of communication at the data
link layer is a frame.
Note

Figure 2.12 Communication at the network layer
A
Physical Physical
Data link
Data link
R1 R3 R4 B
Network
Network
D Header
H
Legend
Datagram
D3 H3
Datagram
D3 H3

network layer is a datagram.
Note

Figure 2.13 Communication at transport layer
A
Physical Physical
Data link
Data link
R1 R3 R4
B
Network
Network
Transport Transport
D Header
H
Legend
Segment
D4 H4
Segment
D4 H4

transport layer is a segmentt, user
datagram, or a packet, depending on the
specific protocol used in this layer.
Note

Figure 2.14 Communication at application layer
A
Physical Physical
Data link
Data link
R1 R3 R4
B
Network
Network
Transport Transport
Application
Application Source Destination Data
D Header
H
Legend
Message
D5 D5
D5 D5
Message

application layer is a message.
Note

2-4 ADDRESSING
Four levels of addresses are used in an internet
employing the TCP/IP protocols: physical
address, logical address, port address, and
application-specific address. Each address is
related to a one layer in the TCP/IP architecture,
as shown in Figure 2.15.

Topics Discussed in the Section
✓ Physical Addresses
✓ Logical Addresses
✓ Port Addresses
✓ Application-Specific Addresses

Figure 2.15 Addresses in the TCP/IP protocol suite

In Figure 2.16 a node with physical address 10 sends a frame to
a node with physical address 87. The two nodes are connected
by a link (a LAN). At the data link layer, this frame contains
physical (link) addresses in the header. These are the only
addresses needed. The rest of the header contains other
information needed at this level. As the figure shows, the
computer with physical address 10 is the sender, and the
computer with physical address 87 is the receiver. The data link
layer at the sender receives data from an upper layer. It
encapsulates the data in a frame. The frame is propagated
through the LAN. Each station with a physical address other
than 87 drops the frame because the destination address in the
frame does not match its own physical address. The intended
destination computer, however, finds a match between the
destination address in the frame and its own physical address.
Example 2.3

Figure 2.16 Example 2.3: physical addresses
Data
87 10
1 packet
accepted
Data
87 10
4

As we will see in Chapter 3, most local area networks use a
48-bit (6-byte) physical address written as 12 hexadecimal
digits; every byte (2 hexadecimal digits) is separated by a
colon, as shown below:
Example 2.4
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address

Figure 2.17 shows a part of an internet with two routers
connecting three LANs. Each device (computer or router) has a
pair of addresses (logical and physical) for each connection. In
this case, each computer is connected to only one link and
therefore has only one pair of addresses. Each router, however,
is connected to three networks. So each router has three pairs of
addresses, one for each connection. Although it may be obvious
that each router must have a separate physical address for each
connection, it may not be obvious why it needs a logical address
for each connection. We discuss these issues in Chapters 11
and 12 when we discuss routing. The computer with logical
address A and physical address 10 needs to send a packet to the
computer with logical address P and physical address 95. We
use letters to show the logical addresses and numbers for
physical addresses, but note that both are actually numbers, as
we will see in later chapters.
Example 2.5

Figure 2.17 Example 2.5: logical addresses
Data
A P
20 10 Data
A P
20 10
Physical
addresses
changed
Data
A P
33 99
Data
A P
33 99
Physical
addresses
changed
Data
A P
95 66 Data
A P
95 66

The physical addresses will change from
hop to hop, but the logical addresses
remain the same.
Note

Figure 2.18 shows two computers communicating via the
Internet. The sending computer is running three processes
at this time with port addresses a, b, and c. The receiving
computer is running two processes at this time with port
addresses j and k. Process a in the sending computer
needs to communicate with process j in the receiving
computer. Note that although both computers are using the
same application, FTP, for example, the port addresses are
different because one is a client program and the other is a
server program, as we will see in Chapter 17.
Example 2.6

A Sender Receiver P
Internet
Figure 2.18 Example 2.6: port numbers
a Data
j
A P
H2
a Data
j
A P
a Data
j
Data
a Data
j
A P
H2
a Data
j
A P
a Data
j
Data

The physical addresses change from
hop to hop, but the logical and port
addresses usually remain the same.
Note

As we will see in future chapters, a port address is a 16-bit
address represented by one decimal number as shown.
Example 2.7
753
A 16-bit port address represented as one single number

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