OSI is a seven-layer network model specified by ISO. It
is formally referred to as the Basic Reference Model
for Open Systems Interconnection. The seven layers are:
- Layer 7 - Application
- This layer is concerned with the application related to the
communication between systems. This layer would convey quality of service
parameters, authenticate users, and verify that the protocol exchanges
are correct (e.g., ensure that a message is not transmitted that would
result in a protocol violation).
- Layer 6 - Presentation
- This is the layer that is responsible for digesting the received
messages and "understanding" them or properly composing messages
that may be understood by the other device. This might include
the formation of a stream of data formatted according to the HTTP
protocol specifications or encoding a message according to an ASN.1
protocol specification.
- Layer 5 - Session
- This layer is responsible for setting up and managing sessions or
dialogs between two applications or services. It relies on the transport
protocol underneath to deliver messages. In the world of IP, this might
be viewed as the TCP APIs or routines that tranmit and receive
UDP packets in an application.
- Layer 4 - Transport
- This is the layer at which differentiation is made between applications
or services on the network from the underlying protocols. In the world
of IP, this equates to TCP and UDP, though TCP also falls a little into
layer 5, as well.
- Layer 3 - Network
- The network layer defines the protocol that allows for communication
between various system and applications on any number of network links.
The Internet Protocol is an example of a protocol that operates
at Layer 3.
- Layer 2 - Datalink
- The datalink layer is the lowest layer at which some defined message
is transmitted. This layer includes protocols like Ethernet. Messages
at this layer contain sufficient information about the destination to
be properly delivered to the proper network interface on a host
on a single single link.
- Layer 1 - Physical
- This layer refers to the physical medium (e.g., fiber optic cable,
twisted-pair cable, coaxial cable, etc.) through which a device
is connected.
