PREPARED BY HARSHIT PATALIA
De facto, de jurie
• Standards fall into two categories: de facto and de jure.
• De facto (Latin for ‘‘from the fact’’) standards are those that have
just happened, without any formal plan. HTTP, the protocol on
which the Web runs, started life as a de facto standard. It was part
of early WWW browsers developed by Tim Berners-Lee at
CERN, and its use took off with the growth of the Web. Bluetooth is
another ex-ample. It was originally developed by Ericsson but now
everyone is using it.
• De jure(Latin for ‘‘by law’’) standards, in contrast, are adopted
through the rules of some formal standardization body.
International standardization authorities are generally divided into
two classes: those established by treaty among national
governments, and those comprising voluntary, non- treaty
organizations. In the area of computer network standards like
ISO, IEEE, ASCII.
The Seven Layers of the OSI Model
7. Application Layer
6. Presentation Layer
5. Session Layer
4. Transport Layer
3. Network Layer
2. Data Link Layer
1. Physical Layer
• The application layer is the OSI
layer that is closest to the user; it
provides network services to the
user’s applications. It differs from
the other layers in that it does not
provide services to any other OSI
layer, but rather, only to
applications outside the OSI model.
Examples of such applications are
spreadsheet programs, word
processing programs, and bank
terminal programs. If you want to
remember Layer 7 in as few words
as possible, think of browsers.
Application layer interacts with application programs and is
the highest level of OSI model.
Application layer contains management functions to
support distributed applications.
Examples of application layer are applications such as file
transfer, electronic mail, remote login etc.
• The presentation layer ensures
that the information that the
application layer of one system
sends out is readable by the
application layer of another
system. If necessary, the
presentation layer translates
between multiple data formats
by using a common format. If
you want to think of Layer 6 in
as few words as possible, think
of a common data format
Presentation layer defines the format in which the data is to
be exchanged between the two communicating entities.
Also handles data compression and data encryption
• As its name implies, the session
layer establishes, manages, and
terminates sessions between two
communicating hosts. The session
layer provides its services to the
presentation layer. It also
synchronizes dialogue between
the two hosts' presentation layers
and manages their data exchange.
If you want to remember Layer 5
in as few words as possible, think
of dialogues and conversations.
Session layer provides mechanism for controlling the dialogue
between the two end systems. It defines how to start, control and
end conversations (called sessions) between applications.
This layer requests for a logical connection to be established on
an end-user’s request.
Any necessary log-on or password validation is also handled by
Session layer is also responsible for terminating the connection.
This layer provides services like dialogue discipline which can be
full duplex or half duplex.
Session layer can also provide check-pointing mechanism such
that if a failure of some sort occurs between checkpoints, all data
can be retransmitted from the last checkpoint.
• The transport layer
Segments and Sequences
data from the sending host's
system and reassembles the
data into a data stream on
the receiving host's system.
Layer 4 is the boundary
protocols and host-layer
protocols. Layer 4 also deals
with Flow Control through
(Windowing) or Window
Negotiation. And is
responsible for the
Reliability of Communication
Purpose of this layer is to provide a reliable mechanism for
the exchange of data between two processes in different
Ensures that the data units are delivered error free.
Ensures that data units are delivered in sequence.
Ensures that there is no loss or duplication of data units.
Provides connectionless or connection oriented service.
Provides for the connection management.
Multiplex multiple connection over a single channel.
• The network layer is a
complex layer that
and path selection
between two host
systems that may be
separated networks. If
you want to remember
Layer 3 in as few
possible, think of path
selection, routing, and
Routers are layer 3 devices
Implements routing of frames (packets) through the
Defines the most optimum path the packet should take from
the source to the destination
Defines logical addressing so that any endpoint can be
Handles congestion in the network.
Facilitates interconnection between heterogeneous
The network layer also defines how to fragment a packet
into smaller packets to accommodate different media.
• The data link layer
provides reliable transit of
data across a physical link.
In so doing, the data link
layer is concerned with
delivery of frames. If you
want to remember Layer 2
in as few words as
possible, think of physical
addressing, and topologies
Switches, Bridges, and NICs are layer 2
Data Link Layer
Data link layer attempts to provide reliable communication
over the physical layer interface.
Breaks the outgoing data into frames and reassemble the
Create and detect frame boundaries.
Handle errors by implementing an acknowledgement and
Implement flow control.
Supports points-to-point as well as broadcast
Supports simplex, half-duplex or full-duplex communication.
• The physical layer defines the
electrical, mechanical, procedural,
and functional specifications for
activating, maintaining, and
deactivating the physical link
between end systems. Such
characteristics as voltage
levels, timing of voltage
changes, physical data
rates, maximum transmission
other, similar, attributes are
defined by physical layer
specifications. If you want to
remember Layer 1 in as few words
as possible, think of signals and
Hubs, cables, and connectors are layer 1
Provides physical interface for transmission of information.
Defines rules by which bits are passed from one system to
another on a physical communication medium.
Covers all - mechanical, electrical, functional and procedural
- aspects for physical communication.
Such characteristics as voltage levels, timing of voltage
changes, physical data rates, maximum transmission
distances, physical connectors, and other similar attributes
are defined by physical layer specifications.
Protocol Data Units (PDU) and Peer to Peer Communication
Detailed encapsulation process
• All communications on a network originate at a source, and are sent to a
• The information sent on a network is referred to as data or data packets. If
one computer (host A) wants to send data to another computer (host B), the
data must first be packaged through a process called encapsulation.
Detailed encapsulation process
Networks must perform the following five conversion steps in order to
1. Build the data.
2. Package the data for end-to-end transport.
3. Add the network IP address to the header.
4. Add the data link layer header and trailer.
5. Convert to bits for transmission.
• Unlike the proprietary networking technologies mentioned earlier, TCP/IP
was developed as an open standard.
• This meant that anyone was free to use TCP/IP. This helped speed up the
development of TCP/IP as a standard.
• Although some of the layers in the TCP/IP model have the same name as
layers in the OSI model, the layers of the two models do not correspond
Some of the common protocols specified by the TCP/IP reference model layers. Some of the most
commonly used application layer protocols include the following:
• File Transfer Protocol (FTP)
• Hypertext Transfer Protocol (HTTP)
• Simple Mail Transfer Protocol (SMTP)
• Domain Name System (DNS)
• Trivial File Transfer Protocol (TFTP)
The common transport layer
• Transport Control Protocol (TCP)
• User Datagram Protocol (UDP)
The primary protocol of the
Internet layer is:
• Internet Protocol (IP)
Networking professionals differ in their opinions on which model to use. Due to the
nature of the industry it is necessary to become familiar with both. Both the OSI and
TCP/IP models will be referred to throughout the curriculum. The focus will be on the
• TCP as an OSI Layer 4 protocol
• IP as an OSI Layer 3 protocol
• Ethernet as a Layer 2 and Layer 1 technology
Remember that there is a difference between a model and an actual protocol that is used
in networking. The OSI model will be used to describe TCP/IP protocols.
Use Ethereal to capture TCP/IP packets wrapped in an Ethernet frame