OSI and TCP/IP reference models in networking

Reference Models
MCA-14-32 Unit I

OSI Reference Model
 This model is based on a proposal developed by the
International Standards Organization (ISO) as a first
step toward international standardization of the
protocols used in the various layers (1983).
 It was revised in 1995.
 The model is called the ISO OSI (Open Systems
Interconnection) Reference Model because it deals
with connecting open systems.
 The OSI model has 7 layers.

OSI Reference Model
 The principles that were applied to arrive at the seven
layers are as follows:
1. A layer should be created where a different abstraction is
needed.
2. Each layer should perform a well-defined function.
3. The function of each layer should be chosen keeping in
view internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the
information flow across the interfaces.
5. The number of layers should be large enough that distinct
functions need not be thrown together in the same layer
out of necessity and small enough that the architecture
does not become unwieldy.

OSI Reference Model
 OSI model itself is not a network architecture because
it does not specify the exact services and protocols to
be used in each layer. It just tells what each layer
should do.

Physical Layer
 The physical layer transmits raw bits over a communication
channel.
 It has to make sure that when one side sends a 1 bit, it is
received by the other side as a 1 bit, not as a 0 bit.
 It deals with
 how many volts should be used to represent a 1 and how many
for a 0,
 How many nanoseconds a bit lasts,
 whether transmission may proceed simultaneously in both
directions,
 how the initial connection is established and how it is
disconnected when both sides are finished,
 how many pins the network connector has and what each pin is
used for.
 The design issues here largely deal with mechanical,
electrical, and timing interfaces, and the physical
transmission medium, which lies below the physical layer.

Functions of Physical Layer
 Physical characteristics of interfaces and media-
defines the characteristics of interface between the
devices and the transmission medium
 Representation of bits- conversion between
representation of bits and corresponding signals
 Transmission rate- number of bits sent each second,
the duration of a bit
 Synchronization of bits- the synchronization between
sender and receiver clocks

Data Link Layer (node to node delivery)
 The main task is to transform a raw transmission facility
(with errors) into a line that appears free of undetected
transmission errors to the network layer.
 It accomplishes this task by having the sender break up the
input data into data frames and transmit the frames
sequentially.
 If the service is reliable, the receiver confirms correct
receipt of each frame by sending back an
acknowledgement frame.
 It does flow control by keeping a fast transmitter from
drowning a slow receiver in data.
 Broadcast networks have an additional issue in the data
link layer: how to control access to the shared channel. A
special sublayer of the data link layer, the medium access
control sublayer, deals with this problem.

Functions of Data Link Layer
 Framing
 Physical addressing
 Flow control
 Error control
 Access control

Network Layer (source to destination delivery)
 This layer controls the operation of the subnet.
 It decides how packets are routed from source to
destination.
 Routes can be based on static tables that are ''wired
into'' the network and rarely changed.
 They can also be determined at the start of each
conversation, for example, a login to a remote
machine.
 They can be highly dynamic, being determined anew
for each packet, to reflect the current network load.

Network Layer
 When a packet has to travel from one network to
another to get to its destination, many problems can
arise.
 The addressing used by the second network may be
different from the first one. The second one may not
accept the packet at all because it is too large. The
protocols may differ, and so on.
 It is up to the network layer to overcome all these
problems to allow heterogeneous networks to be
interconnected.
 In broadcast networks, the routing problem is simple,
so the network layer is often thin or even nonexistent.

Functions of Network Layer
 Logical addressing
 Routing

Transport Layer (process to process delivery)
 The transport layer is a true end-to-end layer, all the
way from the source to the destination.
 In other words, a program on the source machine
carries on a conversation with a similar program on
the destination machine, using the message headers
and control messages.
 In the lower layers, the protocols are between each
machine and its immediate neighbors, and not
between the ultimate source and destination
machines, which may be separated by many routers.
 The basic function of the transport layer is to accept
data from session layer, split it up into smaller units
and pass these to the network layer, and ensure that
the pieces all arrive correctly at the other end.

Transport Layer
 The transport layer determines what type of service to
provide to the session layer, and, ultimately, to the
users of the network.
 The most popular type of transport connection is an
error-free point-to-point channel that delivers
messages or bytes in the order in which they were
sent.

Functions of Transport Layer
 Port addressing
 Segmentation and reassembly
 Connection control
 Flow control
 Error control

Session Layer
 The session layer allows users on different machines
to establish sessions between them.
 Sessions offer various services, including
 dialog control (keeping track of whose turn it is to
transmit),
 token management (preventing two parties from
attempting the same critical operation at the same time),
and
 synchronization (checkpointing long transmissions to
allow them to continue from where they were after a crash).

Functions of Session Layer
 It allows two applications running on different
computers to establish, use and end a connection
called a session.
 It performs name recognition and security.
 It provides synchronization by placing checkpoints in
the data stream.
 It implements dialog control between communicating
processes.

Presentation Layer
 Unlike lower layers, which are mostly concerned with
moving bits around, the presentation layer is
concerned with the syntax and semantics of the
information transmitted.
 In order to make it possible for computers with
different data representations to communicate, the
data structures to be exchanged can be defined in an
abstract way, along with a standard encoding to be
used ''on the wire.''
 The presentation layer manages these abstract data
structures and allows higher-level data structures
(e.g., banking records), to be defined and exchanged.

Functions of Presentation Layer
 Data compression – reduction in the size of data to
achieve faster transmission over the network
 Data encryption – translation of data from a format
used by application layer into a common format and
vice-versa.
 Protocol translation – conversion of data from one
protocol to another to transfer between different
platforms or operating systems.

Application Layer
 The application layer contains a variety of protocols
that are commonly needed by users.
 One widely-used application protocol is HTTP
(HyperText Transfer Protocol), which is the basis for
the World Wide Web. When a browser wants a Web
page, it sends the name of the page it wants to the
server using HTTP. The server then sends the page
back.
 Other application protocols are used for file transfer,
electronic mail, and network news.

Functions of Application Layer
 Mail services - basis for email forwarding and storage
 File transfer and access
 Remote login
 Accessing the world wide web

Fig. Data transmission in OSI Model

TCP/IP Reference Model
 TCP/IP reference model originates from the
grandparent of all computer networks, the ARPANET
and now is used in its successor, the worldwide
Internet.
 The ARPANET was a research network sponsored by
the DoD (U.S. Department of Defense) that connected
hundreds of universities and government installations,
using leased telephone lines.
 When satellite and radio networks were added later,
the existing protocols had trouble interworking with
them, so a new reference architecture was needed.
This architecture later became known as the TCP/IP
Reference Model, after its two primary protocols.
 It was first defined in 1974. A later perspective is given
in 1985.

TCP/IP Reference Model
 DoD wanted connections to remain intact as long as
the source and destination machines were functioning,
even if some of the machines or transmission lines in
between were suddenly put out of operation.
 Furthermore, a flexible architecture was needed since
applications with divergent requirements were
envisioned, ranging from transferring files to real time
speech transmission.

Internet Layer
 This layer permit hosts to inject packets into any
network and have them travel independently to the
destination (potentially on a different network).
 They may even arrive in a different order than they
were sent, in which case it is the job of higher layers to
rearrange them, if in-order delivery is desired.
 The internet layer defines an official packet format and
protocol called IP (Internet Protocol).
 The job of the internet layer is to deliver IP packets
where they are supposed to go.
 Packet routing is clearly the major issue here to avoid
congestion.
 TCP/IP internet layer is similar in functionality to the
OSI network layer

Transport Layer
 This layer is designed to allow peer entities on the
source and destination hosts to carry on a
conversation, just as in the OSI transport layer.
 Two end-to-end transport protocols have been defined
here.
 TCP (Transmission Control Protocol)
 UDP (User Datagram Protocol)

Transport Layer (TCP Protocol)
 TCP is a reliable connection-oriented protocol that
allows a byte stream originating on one machine to be
delivered without error on any other machine in the
internet.
 It fragments the incoming byte stream into discrete
messages and passes each one on to the internet
layer.
 At the destination, the receiving TCP process
reassembles the received messages into the output
stream.
 TCP also handles flow control to make sure a fast
sender cannot swamp a slow receiver with more
messages than it can handle.

Transport Layer (UDP Protocol)
 UDP is an unreliable connectionless protocol for
applications that do not want TCP's sequencing or
flow control and wish to provide their own.
 It is also widely used for one-shot, client-server-type
request-reply queries and applications in which prompt
delivery is more important than accurate delivery, such
as transmitting speech or video.

Application Layer
 It contains all the higher-level protocols such as virtual
terminal (TELNET), file transfer (FTP) and electronic mail
(SMTP), DNS, NNTP and HTTP.
 The virtual terminal protocol allows a user on one machine
to log onto a distant machine and work there.
 The file transfer protocol provides a way to move data
efficiently from one machine to another.
 Electronic mail was originally just a kind of file transfer, but
later a specialized protocol (SMTP) was developed for it.
 Domain Name System (DNS) for mapping host names onto
their network addresses.
 NNTP, the protocol for moving USENET news articles
around
 HTTP, the protocol for fetching pages on the World Wide
Web, and many others.

Fig. Protocols and networks in the TCP/IP model initially

Host-to-Network Layer
 This protocol is not defined and varies from host to
host and network to network.
 It just points out that the host has to connect to the
network using some protocol so it can send IP packets
to it.

A comparison of the OSI and TCP/IP
Reference Models

A critique of the OSI Model and Protocols
 Neither the OSI model and its protocols nor the
TCP/IP model and its protocols are perfect.
 Initially it appeared to many experts in the field that the
OSI model and its protocols were going to take over
the world and push everything else out of their way.
 This did not happen. The model was criticized on the
basis of following reasons:
1. Bad timing
2. Bad technology
3. Bad implementations
4. Bad politics

Bad Timings
 The time at which a standard is established is
absolutely critical to its success.
 David Clark of M.I.T. has a theory of standards that he
calls the apocalypse of the two elephants is shown
below:

Bad Timings
 This figure shows the amount of activity surrounding a
new subject.
 When the subject is first discovered, there is a burst of
research activity in the form of discussions, papers,
and meetings.
 After a while this activity subsides, corporations
discover the subject, and the billion-dollar wave of
investment hits.
 It is essential that the standards be written in the
trough in between the two ''elephants.'‘
 If the interval between the two elephants is very short
(because everyone is in a hurry to get started), the
people developing the standards may get crushed.

Bad Technology
 The second reason is that both the model and the
protocols are flawed.
 The choice of 7 layers was more political than
technical, and two of the layers (session and
presentation) are nearly empty, whereas two other
ones (data link and network) are overfull.
 The OSI model, along with the associated service
definitions and protocols, is extraordinarily complex.
 Some functions, such as addressing, flow control, and
error control, reappear again and again in each layer.
For example, error control must be done in the highest
layer to be effective, repeating it over and over in each
of the lower layers is unnecessary and inefficient.

Bad Implementations
 Due to enormous complexity of the model and the
protocols, the initial implementations were huge,
unwieldy, and slow.
 Everyone who tried them got burned.
 It did not take long for people to associate ''OSI'' with
''poor quality.''
 Although the products improved in the course of time,
the image stuck.

Bad Politics
 On account of the initial implementation, many people,
especially in academia, thought of TCP/IP as part of
UNIX, and UNIX in the 1980s in academia was
considered apple pie.
 OSI, on the other hand, was widely thought to be the
creature of the European telecommunication
ministries, the European Community, and later the
U.S. Government.
 This belief was only partly true.

A critique of the TCP/IP Reference Model
 The TCP/IP model and protocols have their problems
too.
 First, the model does not clearly distinguish the
concepts of service, interface, and protocol. Good
software engineering practice requires differentiating
between the specification and the implementation,
something that OSI does very carefully, and TCP/IP
does not.
 Consequently, the TCP/IP model is not much of a
guide for designing new networks using new
technologies.

 Second, the TCP/IP model is not at all general and is
poorly suited to describing any protocol stack other
than TCP/IP.
 Trying to use the TCP/IP model to describe Bluetooth,
for example, is completely impossible.
 Third, the host-to-network layer is not really a layer at
all in the normal sense of the term as used in the
context of layered protocols.
 It is an interface (between the network and data link
layers). The distinction between an interface and a
layer is crucial, and one should not be sloppy about it.

 Fourth, the TCP/IP model does not distinguish (or
even mention) the physical and data link layers. These
are completely different.
 The physical layer has to do with the transmission
characteristics of copper wire, fiber optics, and
wireless communication.
 The data link layer's job is to delimit the start and end
of frames and get them from one side to the other with
the desired degree of reliability.
 A proper model should include both as separate
layers. The TCP/IP model does not do this.

 Finally, although the IP and TCP protocols were
carefully thought out and well implemented, many of
the other protocols were ad hoc, generally produced
by a couple of graduate students.
 The protocol implementations were then distributed
free, which resulted in their becoming widely used,
deeply entrenched, and thus hard to replace. Some of
them are a bit of an embarrassment now.
 The virtual terminal protocol, TELNET, for example,
was designed for a ten-character per second
mechanical Teletype terminal. It knows nothing of
graphical user interfaces and mice. Nevertheless, 25
years later, it is still in widespread use.

Summary
 In summary, despite its problems, the OSI model
(minus the session and presentation layers) has
proven to be exceptionally useful for discussing
computer networks. In contrast, the OSI protocols
have not become popular.
 The reverse is true of TCP/IP: the model is practically
nonexistent, but the protocols are widely used.

OSI and TCP/IP reference models in networking

 Massachusetts Institute of Technology, A Private
university in Cambridge, Massachusetts
 Sloppy – careless, casual

 SMTP – Simple mail transfer protocol
 telnet – telecommunication network
 DNS – Domain name services/system
 Satnet – Satellite network
 NNTP - Network News Transfer Protocol
 Arpanet – Advanced research project agency network
 Usenet - an early non-centralized computer network
for the discussion of particular topics and the sharing
of files via newsgroups.
 Physical address – 48 bit (first 3 bytes to identify
manufacturer)
 Logical address – 32 bit

Expectation
 Smiling face
 Attendance
 Interactive session
 Enjoy the joy of learning
 Be an observer

OSI and TCP/IP reference models in networking

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