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INTER-NETWORKING, INTRANETWORKING AND EXTRANETWORKING, ROUTING, BRIDGING
OSI/TCP IP LAYERS
Lecture: 4
Instructor Mazhar Hussain
1
TODAY AGENDA
Intra, Inter and Extra Networking
 Bridging and Routing
 OSI Model
 TCP/IP Layers

2
INTRA NETWORKING
An intranet is a private network, accessible
only to an organization's staff.[1][2] Generally a
wide range of information and services from the
organization's internal IT systems are available
that would not be available to the public from
the Internet. A company-wide intranet can
constitute an important focal point of internal
communication and collaboration, and provide a
single starting point to access internal and
external resources. In its simplest form an
intranet is established with the technologies
for local area networks (LANs) and wide area
networks (WANs)
3
INTRA NETWORKING USE



Increasingly, intranets are being used to deliver tools,
e.g. collaboration (to facilitate working in groups and
teleconferencing) or sophisticated corporate directories, sales
andcustomer
relationship
management
tools,
project
management etc., to advance productivity.
Intranets are also being used as corporate culture-change
platforms. For example, large numbers of employees discussing
key issues in an intranet forum application could lead to new
ideas in management, productivity, quality, and other corporate
issues.
In large intranets, website traffic is often similar to public website
traffic and can be better understood by using web metrics
software to track overall activity. User surveys also improve
intranet website effectiveness. Larger businesses allow users
within their intranet to access public internet through firewall
servers. They have the ability to screen messages coming and
going keeping security intact.
4
INTRA NETWORKING BENEFITS
Workforce Productivity
 Time and Communication
 Business operation and management
 Cost effective
 Enhance collaboration
 Promot common corporate culture
 immediate updates
 Supports a distributed computing architecture

5
INTER NETWORKING


Internetworking is the practice of connecting a
computer network with other networks through the
use of gateways that provide a common method of
routing information packets between the networks.
The resulting system of interconnected networks is
called an internetwork, or simply an internet.
The most notable example of internetworking is
the Internet, a network of networks based on many
underlying hardware technologies, but unified by an
internetworking protocolstandard, the Internet
6
Protocol Suite, often also referred to as TCP/IP.
EXTRA NETWORKING
An extranet is a website that allows controlled
access to partners, vendors and suppliers or an
authorized set of customers - normally to a subset
of the information accessible from an
organization's intranet. An extranet is similar to
a DMZ in that it provides access to needed
services for authorised parties, without granting
access to an organization's entire network.
 Historically the term was occasionally also used
in the sense of an two organisation sharing their
internal networks over a VPN.

7
THE CASE FOR BRIDGING

Need a device that can bridge different LANs
Only forward packets to intended recipients
 No broadcast!

Send Packet A
BC
Send Packet A
BC
Bridge
Hub
B
B
8
C
C
BRIDGING THE LANS
Hub
Hub

Bridging limits the size of collision domains
Vastly improves scalability
 Question: could the whole Internet be one bridging domain?


Tradeoff: bridges are more complex than hubs
9
Physical layer device vs. data link layer device
 Need memory buffers, packet processing hardware, routing tables

BRIDGE INTERNALS
Bridge
Inputs
Hub
Outputs
Switch
Fabric
Makes
routing
Memory
buffer
 Bridges decisions
have memory buffers to queue packets
 Bridge is intelligent, only forwards packets to the correct
10
output
 Bridges are high performance, full N x line rate is possible
BRIDGES
Original form of Ethernet switch
 Connect multiple IEEE 802 LANs at layer 2
 Goals
1. Forwarding of frames

Reduce the collision domain
2. Learning of (MAC)
 Complete transparency

Addresses
3. “Plug-and-play,”
Spanning Tree
Algorithm (to handle
self-configuring
loops)
 No
hardware of software changes on hosts/hubs

Should not impact existing LAN operations
11
Hub
ROUTING
 Routing
is the process of selecting best
paths in a network. In the past, the term
routing also meant forwarding network
traffic among networks. However, that
latter function is better described
as forwarding. Routing is performed for
many kinds of networks, including
the
telephone
network
(circuit
switching), electronic data networks (such
as theInternet), and transportation
networks.
12
ROUTING-DELIVERY SEMANTICS
Routing schemes differ in their delivery semantics:
 unicast delivers a message to a single specific node
 broadcast delivers a message to all nodes in the network
 multicast delivers a message to a group of nodes that have
expressed interest in receiving the message
 anycast delivers a message to anyone out of a group of
nodes, typically the one nearest to the source
 geocast delivers a message to a geographic area
13
OSI/TCP IP LAYERS
14
7 LAYER OSI MODEL
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HISTORY
Rapid growth of computer networks caused
compatibility problems
 ISO recognized the problem and released the OSI
model in 1984
 OSI stands for Open Systems Interconnection
and consists of 7 Layers
 The use of layers is designed to reduce complexity
and make standardization easier

16
7 LAYERS OF THE OSI MODEL
Layer
Responsible For:
7.) Application Provides Services to User Apps
6.) Presentation Data Representation
5.) Session
Communication Between Hosts
4.) Transport
Flow Ctrl, Error Detection/Correction
3.) Network
End to End Delivery, Logical Addr
2.) Data Link
Media Access Ctrl, Physical Addr
1.) Physical
17
Medium, Interfaces, Puts Bits on Med.
EXAMPLES
Layer
Example
7.) Application HTTP, FTP, SMTP
6.) Presentation ASCII, JPEG, PGP
5.) Session
BOOTP, NetBIOS, DHCP, DNS
4.) Transport
TCP, UDP, SPX
3.) Network
IP, IPX, ICMP
2.) Data Link
Ethernet, Token Ring, Frame Relay
1.) Physical
Bits, Interfaces, Hubs
18
MNEMONICS
(A)ll
7.) (A)pplication
(A)way
(P)eople
6.) (P)resentation (P)izza
(S)eem
5.) (S)ession
(S)ausage
(T)o
4.) (T)ransport
(T)hrow
(N)eed
3.) (N)etwork
(N)ot
(D)ata
2.) (D)ata Link
(D)o
(P)rocessing
1.) (P)hysical
(P)lease
19
FLAT ADDRESSING
Flat addressing schemes do not provide anything
other than a unique identifier. They provide no
real information about where the object being
addressed resides.
 Example: SSN# (may provide insight to where
the person was born, but not to where they are
now)

20
HIERARCHICAL ADDRESSING
Hierarchical addressing schemes provide layers
or a hierarchy to the address that provide
information about where the addressed object
exists within the hierarchy.
 Example: phone numbers (area code, local prefix,
and four digit number unique to that area
code/prefix combination).

21
TALKING TO EVERYONE
Special kinds of addresses exist at both layer #2
and #3 called broadcast addresses
 Typically network devices are interested in only
traffic addressed directly for them and any traffic
addressed with the destination address set to
broadcast
 If they are paying attention to other traffic, they
are said to be in promiscuous mode

22
ENCAPSULATION
Data exists at each layer contained within a unit
called a Protocol Data Unit (PDU).
 PDU’s are referred two ways: N-PDU, and by
special names.
 The process by which data moves between PDU
types is called Encapsulation
 PDU move through interfaces between layers
using Service Access Points (SAP)

23
PDU’S AND THE OSI MODEL
Layer
7.) Application
PDU Name
Data
6.) Presentation Data
5.) Session
Data
4.) Transport
Segment
3.) Network
Packet
2.) Data Link
Frame
1.) Physical
Bits
24
LAYER 1: THE PHYSICAL LAYER
Defines physical medium and interfaces
 Determines how bits are represented
 Controls transmission rate & bit synchronization
 Controls transmission mode: simplex, halfduplex, & full duplex
 PDU: Bits
 Devices: hubs, cables, connectors, etc…

25
LAYER 2: THE DATA LINK LAYER
PDU: Frames
 Keeps Link alive & provides connection for upper
layer protocols
 Based on physical (flat) address space
 Physical addresses are fixed and don’t change
when the node is moved
 Medium/media access control

26
THE DATA LINK LAYER (CONT.)
Flow control and error detection/correction at the
frame level. Think collisions…
 Topology
 Ex: Ethernet, Token Ring, ISDN
 Sublayers: MAC (framing, addressing, & MAC) &
LLC (logical link control – gives error control &
flow control)
 Devices: switches, bridges, NIC’s

27
LAYER 3: THE NETWORK LAYER
PDU: Packet
 End to end delivery of packets
 Creates logical paths
 Path determination (routing)
 Hides the lower layers making things hardware
independent
 Uses logical hierarchical addresses

28
THE NETWORK LAYER (CONT.)
Logical hierarchical addresses do change when a
node is moved to a new subnet
 Devices: routers, firewalls

29
LAYER 4: THE TRANSPORT LAYER
 PDU:
Segment
 Service Point Address (more often called a
port) used to track multiple sessions
between the same systems. SPA’s are
used to allow a node to offer more than
one service (i.e. it could offer both mail
and web services)
 This layer is why you have to specify TCP
or UDP when dealing with TCP/IP
30
THE TRANSPORT LAYER (CONT.)
Must reassemble segments into data using
sequence numbers
 Can use either connectionless or connection
oriented sessions
 Connectionless sessions rely on upper layer
protocols for error control and are often used for
faster less reliable links
 Ex: UDP (used by things like NFS & DNS)

31
THE TRANSPORT LAYER (CONT.)
 Connection
oriented sessions require the
sender to first request a connection, the
receiver to acknowledge the connection,
and that they negotiate how much data
can be sent/received before its reception is
acknowledged
 Uses acknowledgements & retransmission
for error correction
 Example: TCP (used by things like telnet,
http)
32
LAYER 5: THE SESSION LAYER
PDU: Data (from here on up)
 Sometimes called the dialog controller, this layer
establishes, maintains, and terminates sessions
between applications
 Sets duplex between applications
 Defines checkpoints for acknowledgements
during sessions between applications

33
THE SESSION LAYER (CONT.)
 Provides
atomization – Multiple
connections can be treated as one virtual
session. If one fails or is terminated, all
should be terminated.
 Identifies raw data as either application
data or session control information
 Uses fields provided by layers 3 & 4 to
track dialogs between applications /
services
 Provides translations for naming services
 Ex: RPC, X-Windows, LDAP, NFS
34
LAYER 6: THE PRESENTATION LAYER
Data formatting, translation, encryption, and
compression
 Ex: ASCII, EBCDIC, HTML, JPEG

35
LAYER 7: THE APPLICATION LAYER
Provides communication services to applications
 Ex: HTTP, FTP, SMTP

36
THE PRACTICAL BENEFITS OF
UNDERSTANDING THE OSI MODEL
Helps with packet analysis
 Helps foresee problems
 Aides in network design (especially on large scale
networks)

37
NETWORK DESIGN & ADMIN ISSUES
Examining network protocols and how they
relate to the OSI model help aide network
administers design networks and help admins
troubleshoot strange behavior.
 If you don’t understand what mechanisms your
network is using to communicate, you are more
likely to introduce new problems while trying to
fix old ones.

38
EXAMPLE #1
Admin wants to play around with DHCP so they
put the machines that they want to use on
“private IP addresses”.
 What will happen to “normal” DHCP users?

39
EXAMPLE #2
Network congestion: Admin notices that he is
seeing to much traffic on his network. He decides
to break his network in two using a router.
 What are some potential problems associated
with this?
 What might be some better solutions?

40
TCP/IP MODEL
Much older than OSI model
 Consists of 4 layers instead of 7
 TCP/IP model can be mapped to the OSI model

41
TCP/IP VS OSI
TCP/IP
Application
OSI
Transport
Application
Presentation
Session (Layers 7-5)
Transport (Layer 4)
Internet
Network (Layer 3)
Network Interface
Data Link
Physical (Layers 1-2)
42
IEEE STANDARDS
IEEE project 802 started in 1985
 Adopted by ANSI in 1987
 Recognized as an international standard by the
ISO as ISO 8802
 Deals with layers 1 & 2

43
IEEE STANDARDS (CONT.)
At the data link layer (layer 2), defines MAC and
LLC sublayers
 LLC covers media independent topics (802.2 is
the LLC standard)
 MAC topics are dependent on media (802.3,
802.11, 802.5)
 At the physical layer (layer 1), defines a PMI and
PMD

44
COMPARISON AND
CONTRAST BETWEEN THE
OSI AND TCP/IP MODEL
45
INTRODUCTION
 This
presentation would discuss some
comparison and contrast between the 2
main reference models which uses the
concept of protocol layering.
 Open
System Interconnection Model (OSI)
 Transport Control Protocol /Internet
Protocol (TCP/IP)
46
INTRODUCTION

The topics that we will be discussing would be
based on the diagram below.
OSI
TCP / IP
Application (Layer7)
Presentation (Layer6)
Application
Session (Layer 5)
Transport (Layer 4)
Transport
Network (Layer 3)
Internet
Data Link (Layer 2)
Physical (Layer 1)
Physical
47
OUTLINE
Compare the protocol layers that correspond to
each other.
 General Comparison

Focus of Reliability Control
 Roles of Host system
 De-jure vs. De-facto

48
THE UPPER LAYERS
OSI
TCP / IP
Application (Layer7)
Presentation (Layer6)
Application
Session (Layer 5)
Session
Presentation
Application
49
THE SESSION LAYER
The Session layer permits two parties to hold
ongoing communications called a session across a
network.
 Not found in TCP/IP model
 In TCP/IP,its characteristics are provided by the
TCP protocol.
(Transport Layer)
50
THE PRESENTATION LAYER
The Presentation Layer handles data
format information for networked
communications. This is done by
converting data into a generic format that
could be understood by both sides.
 Not found in TCP/IP model
 In TCP/IP, this function is provided by the
Application Layer.
e.g. External Data Representation Standard (XDR)
Multipurpose Internet Mail Extensions (MIME)
51
THE APPLICATION LAYER
The Application Layer is the top layer of the
reference model. It provides a set of interfaces for
applications to obtain access to networked services
as well as access to the kinds of network services
that support applications directly.


OSI
TCP/IP
- FTAM,VT,MHS,DS,CMIP
- FTP,SMTP,TELNET,DNS,SNMP
Although the notion of an application process is
common to both, their approaches to constructing
application entities is different.
52
APPROACHES USE IN CONSTRUCTING
APPLICATION ENTITIES

The diagram below provides an overall view on
the methods use by both the OSI and TCP/IP
model.
53
ISO APPROACH
 Sometime
called Horizontal Approach
 OSI asserts that distributed applications
operate over a strict hierarchy of layers
and are constructed from a common tool
kit of standardized application service
elements.
 In OSI, each distributed application
service selects functions from a large
common “toolbox” of application service
element (ASEs) and complements these
with application service elements that
perform functions specific to given enduser service .
54
TCP/IP APPROACH
 Sometime
called Vertical Approach
 In TCP/IP, each application entity is
composed of whatever set of function it
needs beyond end to end transport to
support a distributed communications
service.
 Most of these application processes builds
on what it needs and assumes only that
an underlying transport mechanism
(datagram or connection) will be provided.
55
TRANSPORT LAYER
OSI
Transport (Layer 4)

TCP / IP
Transport (TCP/UDP)
The functionality of the transport layer is to
provide “transparent transfer of data from a source
end open system to a destination end open system”
(ISO / IEC 7498: 1984).
56
TRANSPORT LAYER

Transport is responsible for creating and
maintaining the basic end-to-end connection
between communicating open systems, ensuring
that the bits delivered to the receiver are the
same as the bits transmitted by the sender; in
the same order and without modification, loss or
duplication
57
OSI TRANSPORT LAYER
 It
takes the information to be sent and
breaks it into individual packets that are
sent and reassembled into a complete
message by the Transport Layer at the
receiving node
 Also provide a signaling service for the
remote node so that the sending node is
notified when its data is received
successfully by the receiving node
58
OSI TRANSPORT LAYER

Transport Layer protocols include the capability
to acknowledge the receipt of a packet; if no
acknowledgement is received, the Transport
Layer protocol can retransmit the packet or timeout the connection and signal an error
59
OSI TRANSPORT LAYER
 Transport
protocols can also mark packets
with sequencing information so that the
destination system can properly order the
packets if they’re received out-of-sequence
 In addition, Transport protocols provide
facilities for insuring the integrity of
packets and requesting retransmission
should the packet become garbled when
routed.
60
OSI TRANSPORT LAYER

Transport protocols provide the capability for
multiple application processes to access the
network by using individual local addresses to
determine the destination process for each data
stream
61
TCP/IP TRANSPORT LAYER
Defines two standard transport protocols: TCP
and UDP
 TCP implements a reliable data-stream protocol



connection oriented
UDP implements an unreliable data-stream

connectionless
62
TCP/IP TRANSPORT LAYER
TCP provides reliable data transmission
 UDP is useful in many applications



eg. Where data needs to be broadcasted or
multicasted
Primary difference is that UDP does not
necessarily provide reliable data transmission
63
TCP/IP TRANSPORT LAYER

Many programs will use a separate TCP connection
as well as a UDP connection
64
TCP/IP TRANSPORT LAYER

TCP is responsible for data recovery

by providing a sequence number with each packet
that it sends
TCP requires ACK (ackowledgement) to ensure
correct data is received
 Packet can be retransmitted if error detected

65
TCP/IP TRANSPORT LAYER

Use of ACK
66
TCP/IP TRANSPORT LAYER

Flow control with Window

via specifying an acceptable range of sequence
numbers
67
TCP/IP TRANSPORT LAYER
TCP and UDP introduce the concept of ports
 Common ports and the services that run on them:






FTP
telnet
SMTP
http
POP3
21 and 20
23
25
80
110
68
TCP/IP TRANSPORT LAYER
 By
specifying ports and including port
numbers with TCP/UDP data,
multiplexing is achieved
 Multiplexing allows multiple network
connections to take place simultaneously
 The port numbers, along with the source
and destination addresses for the data,
determine a socket
69
COMPARING TRANSPORT FOR BOTH
MODELS
 The
features of UDP and TCP defined at
TCP/IP Transport Layer correspond to
many of the requirements of the OSI
Transport Layer. There is a bit of bleed
over for requirements in the session layer
of OSI since sequence numbers, and port
values can help to allow the Operating
System to keep track of sessions, but most
of the TCP and UDP functions and
specifications map to the OSI Transport
Layer.
70
COMPARING TRANSPORT FOR BOTH
MODELS
 The
TCP/IP and OSI architecture models
both employ all connection and
connectionless models at transport layer.
However, the internet architecture refers
to the two models in TCP/IP as simply
“connections” and datagrams. But the OSI
reference model, with its penchant for
“precise” terminology, uses the terms
connection-mode and connection-oriented
for the connection model and the term
connectionless-mode for the
connectionless model.
71
NETWORK VS. INTERNET
OSI
Network (Layer 3)
TCP / IP
Internet
 Like
all the other OSI Layers, the network
layer provides both connectionless and
connection-oriented services. As for the
TCP/IP architecture, the internet layer is
exclusively connectionless.
72
NETWORK VS. INTERNET
 X.25
Packet Level Protocol – OSI’s
Connection-oriented Network Protocol
The CCITT standard for X.25 defines the
DTE/DCE interface standard to provide access to
a packet-switched network. It is the network
level interface, which specifies a virtual circuit
(VC) service. A source host must establish a
connection (a VC) with the destination host
before data transfer can take place. The network
attempts to deliver packets flowing over a VC in
sequence.
73
NETWORK VS. INTERNET
 Connectionless

Network Service
Both OSI and TCP/IP support a connectionless
network service: OSI as an alternative to
network connections and TCP/IP as the only
way in use.
 Internetworking

Protocols
OSI’s CLNP (ISO/IEC 8473: 1993) is
functionally identical to the Internet’s IP (RPC
791). Both CLNP and IP are best-effortdelivery network protocols. Bit niggling aside,
they are virtually identical. The major
difference between the two is that CLNP
accommodates variable-length addresses,
whereas IP supports fixed, 32-bit address.
74
NETWORK VS. INTERNET
 Internet

(IP) Addresses
The lnternet network address is more
commonly called the “IP address.” It consists of
32 bits, some of which are allocated to a highorder network-number part and the remainder
of which are allocated to a low-order hostnumber part. The distribution of bits - how
many form the network number, and how
many are therefore left for the host number can be done in one of three different ways,
giving three different classes of IP address
75
NETWORK VS. INTERNET
 OSI

Network Layer Addressing
ISO/IEC and CCITT jointly administer the
global network addressing domain. The initial
hierarchical decomposition of the NSAP
address is defined by (ISO/IEC 8348). The
standard specifies the syntax and the
allowable values for the high-order part of the
address - the Initial Domain Part (IDP), which
consists of the Authority and Format Identifier
(AFI) and the Initial Domain Identifier (IDI) but specifically eschews constraints on or
recommendations concerning the syntax or
semantics of the domain specific part (DSP).
76
NETWORK VS. INTERNET
 OSI

Routing Architecture
End systems (ESs) and intermediate systems
(ISs) use routing protocols to distribute
(“advertise”) some or all of the information
stored in their locally maintained routing
information base. ESs and ISs send and
receive these routing updates and use the
information that they contain (and information
that may be available from the local
environment, such as information entered
manually by an operator) to modify their
routing information base.
77
NETWORK VS. INTERNET
 TCP/IP

Routing Architecture
The TCP/IP routing architecture looks very
much like the OSI routing architecture. Hosts
use a discovery protocol to obtain the
identification of gateways and other hosts
attached to the same network (subnetwork).
Gateways within autonomous systems (routing
domains) operate an interior gateway protocol
(intradomain IS-IS routing protocol), and
between autonomous systems, they operate
exterior or border gateway protocols
(interdomain routing protocols). The details
are different but the principles are the same.
78
DATA LINK / PHYSICAL VS. SUBNET
OSI
TCP / IP
Data Link (Layer 2)
Physical
Physical (Layer 1)
 Data link layer
 The function of the Data Link Layer is “provides for the control of
the physical layer, and detects and possibly corrects errors which
may occur” (IOS/IEC 7498:1984). In another words, the Data Link
Layer transforms a stream of raw bits (0s and 1s) from the physical
into a data frame and provides an error-free transfer from one node
to another, allowing the layers above it to assume virtually error79
free transmission
DATA LINK / PHYSICAL VS. SUBNET
 Physical layer
 The function of the Physical Layer is to provide
“mechanical, electrical, functional, and procedural
means to activate a physical connection for bit
transmission” (ISO/IEC 7498:1984). Basically, this
means that the typical role of the physical layer is to
transform bits in a computer system into
electromagnetic (or equivalent) signals for a particular
transmission medium (wire, fiber, ether, etc.)
80
DATA LINK / PHYSICAL VS. SUBNET
 Comparing
to TCP/IP

These 2 layers of the OSI correspond directly to the subnet layer
of the TCP/IP model.

Majority of the time, the lower layers below the Interface or
Network layer of the TCP/IP model are seldom or rarely
discussed. The TCP/IP model does nothing but to high light the
fact the host has to connect to the network using some protocol
so it can send IP packets over it. Because the protocol used is
not defines, it will vary from host to host and network to network
81
DATA LINK / PHYSICAL VS. SUBNET

Comparing to TCP/IP
 After much deliberation by organizations, it was
decided that the Network Interface Layer in the
TCP/IP model corresponds to a combination of the
OSI Data Link Layer and network specific functions
of the OSI network layer (eg IEEE 203.3).

Since these two layers deal with functions that are so
inherently specific to each individual networking
technology, the layering principle of grouping them
together related functions is largely irrelevant.
82
GENERAL COMPARISON
Focus of Reliability Control
 Roles of Host System
 De-jure vs. De-facto

83
FOCUS OF RELIABILITY CONTROL
Implementation of the OSI model places emphasis on
providing a reliable data transfer service, while the
TCP/IP model treats reliability as an end-to-end
problem.
 Each layer of the OSI model detects and handles errors,
all data transmitted includes checksums. The transport
layer of the OSI model checks source-to-destination
reliability.
 In the TCP/IP model, reliability control is concentrated
at the transport layer. The transport layer handles all
error detection and recovery. The TCP/IP transport
layer uses checksums, acknowledgments, and timeouts84
to control transmissions and provides end-to-end

ROLES OF HOST SYSTEM

Hosts on OSI implementations do not handle
network operations (simple terminal), but
TCP/IP hosts participate in most network
protocols. TCP/IP hosts carry out such functions
as end-to-end verification, routing, and network
control. The TCP/IP internet can be viewed as a
data stream delivery system involving intelligent
hosts.
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DE-JURE VS. DE-FACTO (OSI)
 OSI
 Standard legislated by official recognized body. (ISO)
 The OSI reference model was devised before the protocols were
invented. This ordering means that the model was not biased
toward one particular set of protocols, which made it quite
general. The down side of this ordering is that the designers did
not have much experience with the subject and did not have a
good idea of which functionality to put in which layer.
 Being general,the protocols in the OSI model are better hidden
than in the TCP/IP model and can be replaced relatively easily
as the technology changes.
 Not so widespread as compared with TCP/IP. (complex , costly)
 More commonly used as teaching aids.
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DE-JURE VS. DE-FACTO (TCP/IP)
 TCP/IP
 Standards adopted due to widespread use. (Internet)
 The protocols came first, and the model was really just a
description of the existing protocols. There was no problem
with the protocols fitting the model, but it is hardly
possible to be use to describe other models.
 “Get the job done" orientation.
Over the years it has handled most challenges by growing
to meet the needs.
 More popular standard for internetworking for several
reasons :



relatively simple and robust compared to alternatives such as OSI
available on virtually every hardware and operating system platform
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(often free)
the protocol suite on which the Internet depends.