B
Appendix
197
OSI Model
Overview
The
Open
Systems
Interconnection
(OSI)
model
defines
a
framework
for
inter
‐
computer
communications.
It
categorizes
different
protocols
for
a
great
variety
of
network
applications
into
seven
smaller,
more
manageable
layers.
The
model
describes
how
data
from
an
application
in
one
computer
can
be
transferred
through
a
network
medium
to
an
application
on
another
computer.
Control
of
data
traffic
is
passed
from
one
layer
to
the
next,
starting
at
the
application
layer
in
one
computer,
proceeding
to
the
bottom
layer,
traversing
over
the
medium
to
another
computer
and
then
delivering
up
to
the
top
of
the
hierarchy.
Each
layer
handles
a
certain
set
of
protocols,
so
that
the
tasks
for
achieving
an
application
can
be
distributed
to
different
layers
and
be
implemented
independently.
The
model
is
relevant
to
understanding
many
aspects
of
the
SEG,
such
as
ARP
and
services.
Layer functions
The
different
layers
perform
the
following
functions:
Layer
7
–
Application
Layer
Defines
the
user
interface
that
supports
applications
directly.
Protocols:
HTTP,
FTP,
TFTP.
DNS,
SMTP,
Telnet,
SNMP
and
similar.
The
ALGs
operate
at
this
level.
Layer
6
–
Presentation
Layer
Translates
the
various
applications
to
uniform
network
formats
that
the
rest
of
the
layers
can
understand.
Layer
5
–
Session
Layer
Establishes,
maintains
and
terminates
sessions
across
the
network.
Protocols:
NetBIOS,
RPC
and
similar.
Table 1. The 7 layers of the OSI model
Layer number
Layer purpose
Layer 7
Application
Layer 6
Presentation
Layer 5
Session
Layer 4
Transport
Layer 3
Network
Layer 2
Data-Link
Layer 1
Physical
