NETWORKING - Hacking: The Art of Exploitation - page 196

0x410 OSI Model

When two computers talk to each other, they need to speak the same lan-
guage. The structure of this language is described in layers by the OSI model.
The OSI model provides standards that allow hardware, such as routers and
firewalls, to focus on one particular aspect of communication that applies to
them and ignore others. The OSI model is broken down into conceptual
layers of communication. This way, routing and firewall hardware can focus
on passing data at the lower layers, ignoring the higher layers of data encap-
sulation used by running applications. The seven OSI layers are as follows:

Physical layer This layer deals with the physical connection between
two points. This is the lowest layer, whose primary role is communicating
raw bit streams. This layer is also responsible for activating, maintaining,
and deactivating these bit-stream communications.
Data-link layer This layer deals with actually transferring data between
two points. In contrast with the physical layer, which takes care of send-
ing the raw bits, this layer provides high-level functions, such as error
correction and flow control. This layer also provides procedures for acti-
vating, maintaining, and deactivating data-link connections.
Network layer This layer works as a middle ground; its primary role is
to pass information between the lower and the higher layers. It provides
addressing and routing.
Tr an sp o r t lay e r This layer provides transparent transfer of data between
systems. By providing reliable data communication, this layer allows the
higher layers to never worry about reliability or cost-effectiveness of data
transmission.
Session layer This layer is responsible for establishing and maintaining
connections between network applications.
Presentation layer This layer is responsible for presenting the data to
applications in a syntax or language they understand. This allows for
things like encryption and data compression.
Application layer This layer is concerned with keeping track of the
requirements of the application.

When data is communicated through these protocol layers, it's sent in
small pieces called packets. Each packet contains implementations of these
protocol layers. Starting from the application layer, the packet wraps the pre-
sentation layer around that data, which wraps the session layer, which wraps
the transport layer, and so forth. This process is called encapsulation. Each
wrapped layer contains a header and a body. The header contains the pro-
tocol information needed for that layer, while the body contains the data for
that layer. The body of one layer contains the entire package of previously
encapsulated layers, like the skin of an onion or the functional contexts
found on a program's stack.