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Introduction to CCNP on OSI and TCP/IP Model
BN1024 – Demo PPT
Demo CCNP

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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)

OSI- TCP/IP
The topics that we will be discussing would be based on the diagram below.
OSI TCP / IP
Application (Layer7) Application
Presentation (Layer6) Application
Session (Layer 5) Application
Transport (Layer 4) Transport
Network (Layer 3) Internet
Data Link (Layer 2) Subnet
Physical (Layer 1) Subnet

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) Application
Presentation (Layer6) Application
Session (Layer 5) Application
• 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 th
at 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 OSIand 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 complement
s these with application service elements that perform functions specific
to given end-user service .

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
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).
OSI TCP / IP
Transport (Layer 4) Transport (TCP/UDP)

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

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 succes
sfully 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
•UDP implements an unreliable data-stream
•connectionless

• 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 provid
e reliable data transmission

Many programs will use a separate TCP connection as well as a UDP connection

•TCP is responsible for data recovery
•by providing a sequence number with each packet that it sends
•TCP requires ACK (acknowledgement) to ensure correct data is received
•Packet can be retransmitted if error detected

Use of ACK

•Flow control with Window
•via specifying an acceptable range of sequence numbers

•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

•By specifying ports and including port numbers with
TCP/UDP data, multiplexingis 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 connectionless model.

Network vs. 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.
OSI TCP / IP
Network (Layer 3) 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.

•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, whereas IP supports fixed, 32-bit address.

•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-
numberpart and the remainder of which are allocated to a low-order host-
numberpart. 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

•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).

•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 inf
ormation base.

•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 (intra-domain IS-IS routing protocol), and
between autonomous systems, they operate exterior or border gateway
protocols (inter-domain routing protocols). The details are different but the
principles are the same.

Data link / Physical vs. Subnet
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
OSI TCP / IP
Data Link (Layer 2) Subnet
Physical (Layer 1) 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.)

•Comparing to TCP/IP
•These 2 layers of the OSI correspond directlyto 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

•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 transmissions and provides end-to-end verification.

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)
•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 setoff 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 wide spread 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)
•the protocol suite on which the Internet depends.

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