Comparison and
Contrast between the
OSI and TCP/IP Model
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)
Introduction
Thetopics that we will be
discussing would be based on the
diagram
OSI below. TCP / IP
Application (Layer7)
Application
Presentation (Layer6)
Session (Layer 5)
Transport (Layer 4) Transport
Network (Layer 3) Internet
Data Link (Layer 2)
Subnet
Physical (Layer 1)
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
The Upper Layers
OSI TCP / IP
Application (Layer7)
Presentation (Layer6) Application
Session (Layer 5)
Session
Presentation
Application
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)
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)
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 - FTAM,VT,MHS,DS,CMIP
TCP/IP - FTP,SMTP,TELNET,DNS,SNMP
Although the notion of an application process
is common to both, their approaches to
constructing application entities is different.
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.
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
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.
Transport Layer
OSI TCP / IP
Transport (Layer 4) Transport (TCP/UDP)
Thefunctionality 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).
Transport Layer
Transportis 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
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
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 time-out
the connection and signal an error
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.
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
TCP/IP Transport Layer
Defines two standard transport
protocols: TCP and UDP
TCP implements a reliable data-
stream protocol
connection oriented
UDPimplements an unreliable
data-stream
connectionless
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
TCP/IP Transport Layer
Manyprograms will use a separate
TCP connection as well as a UDP
connection
TCP/IP Transport Layer
TCPis 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
TCP/IP Transport Layer
Use of ACK
TCP/IP Transport Layer
Flow control with Window
via specifying an acceptable range of
sequence numbers
TCP/IP Transport Layer
TCP and UDP introduce the
concept of ports
Common ports and the services
that run on them:
FTP 21 and 20
telnet 23
SMTP 25
http 80
POP3 110
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
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.
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
Network vs. Internet
OSI TCP / IP
Network (Layer 3) 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.
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.
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-effort-
delivery network protocols. Bit niggling
aside, they are virtually identical. The major
difference between the two is that CLNP
accommodates variable-length addresses,
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 high-order network-number
part and the remainder of which are
allocated to a low-order host-number 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
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).
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.
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.
Data link / Physical vs.
Subnet
OSI TCP / IP
Data Link (Layer 2)
Subnet
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 error-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.)
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
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.
General Comparison
Focus of Reliability Control
Roles of Host System
De-jure vs. De-facto
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 timeouts to control
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.
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.
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 (often free)
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