Basics :
Encapsulation and De-Encapsulation Process
Note: We are talking about TCP/IP model NOT OSI model which uses 7 layers
Application Layer ----> Transport Layer
All transmissions start at the application Layer . The application on the
local computer (client) initiates a request destined for the application
installed on some other remote host (server). That request CANNOT be sent
directly on to the wire (layer 1). Instead, it must be processed by ALL the
layers in between (layers 4 through 2) before it reaches the media. This way, the
request from the application is sent down to the transport layer (layer 4) for
processing. Application will choose one of the two common protocols used in the
layer 4: either TCP, for reliable connections, or UDP for un-reliable
connections.
Transport Layer ----> Internet Layer
The transport layer, upon receiving the request from the
application layer, will process it according to what it has been designed to
do, and is going to stick a so called header to the incoming data (for the
details regarding all headers please, see the next lesson). This header will be
understood and processed by the layer 4 at the receiving host (remote
computer). This segment (as we call it a 'segment' now), is sent down to layer
3 for processing.
Internet Layer ----> Network Interface (Data-Link Layer)
The internet layer will process the incoming data from the
layer 4 (which now consists of data or payload from layer 5 + layer 4 header)
and will stick its own, layer 3 header, to the payload coming from the upper
layers. Once it's done, it will send the packet (as we call it a 'packet' at
this layer) down to the layer 2 for processing.
Data-Link Layer ==> Network Interface (Physical Layer)
The Layer 2, similarly to the previous layers (layer 4 and
layer 3), is going to process the incoming packet according to the functions
designed for this layer. Then, it will attach its own layer 2 header, and is
going to send the frame (the name given to the layer 2 protocol data unit) down
to the layer 1.
Physical Layer = Signaling
The layer 1 is going to transmit bits, 1s and 0s. Bits is
the name given to protocol data unit at this layer according to the OSI model
which is the reference for all models). Technically, Layer 1 is common for any
networking model and follows IEEE specifications.
The process described above, attaching the controlling
information in the forms of the headers, is called encapsulation.
Once the bits arrive at the destination computer (host), the
reverse process takes place which is called de-encapsulation.
Network Interface (Physical Layer) ----> Data-Link Layer
The layer 1 is going to accept incoming bits and send them
up to the layer 2 for processing.
Network Interface (Data-Link Layer) ----> Internet Layer
The layer 2 piece of software can properly interpret the header
information (control information) initially attached by the sender's layer 2
process. So, it reads the layer 2 header, then strips this off, and the content
of the frame (without the layer 2 header it's called a packet now), is sent up
to the layer 3 for processing.
Internet Layer ----> Transport Layer
The layer 3 is going to perform similar actions that the
layer 2 just did. It reads and processes the layer 3 header in the packet.
Notice, that only layer 3 process understands the layer 3 header. Then, it
removes the layer 3 header and sends the content of the packet (data without
layer 2 and layer 3 headers is called a segment) up to the layer 4.
Transport Layer ----> Application Layer
I'm sure you have already guessed what is going to happen
next. Yes, the transport layer will read the layer 4 header, which consists of
the instructions what to do next. Then, upon stripping off the layer 4 header
it will send what was originally created (data request from the client
software) to the appropriate process/application at the receiving host (server
application).
When the server sends the reply back to the client, the
whole encapsulation process will occur again. The client receives the reply
from the server, and De-encapsulates the incoming data like explained above.
And this whole process is repeated back and forth until all
data has been exchanged.
understood sir. .
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