TCP/IP - Layered Architecture

The Transmission Control Protocol/Internet Protocol (TCP/IP) model describes a set of general design guidelines and implementations of specific networking protocols to enable computers to communicate over a network. Layers in the TCP/IP model :

* 1 Network Interface (Physical) Layer : The layer is used to move packets between the Internet Layer interfaces of two different hosts over a physical medium such as a fiber cable or a wire.
* 2 Internet Layer : Internet Protocol performs two basic functions:

• Host addressing and identification: Each host is assigned a valid Internet Protocol Address which is 32-bits long such as 202.144.216.219
  
• Packet routing: This is the basic task of getting packets of data (datagrams) from source to destination by sending them to the next network node (router) closer to the final destination.

* 3 Transport Layer : The layer's responsibilities include end-to-end message transfer capabilities independent of the underlying network, along with error control, flow control, congestion control (avoiding huge data on a particular single route/channel), and application addressing (port numbers).
* 4 Application Layer : The Application Layer refers to the higher-level protocols used by most applications for network communication. Examples of application layer protocols include the File Transfer Protocol (FTP) and the Hyper Text Transmission Protocol (HTTP).

ENCAPSULATION :

• In TCP/IP, as data is being sent from one computer, it will pass from the top layer to the bottom.
• On the receiving end, the data will then be rebuilt from the bottom layer to the top. You can view an example of this process below.
• Each layer a packet of information travels through adds what is called a header. Each layer a sending packet passes through gains another header.
• When the packet is being rebuilt on the receiving end, each header is unpackaged the same way.

EXAMPLE :

• Your browser works at the application layer and accepts the initial datagram, say a web page to be sent across the Internet and adds the appropriate header describing the protocol used.
• The transport layer protocol and the appropriate protocol header is added to the datagram. This controls many of the aspects in the management and initiation of communication between the two hosts.
  
• In the network layer, routers offer the functionality for the datagram to hop from source to the destination, one hop at a time. This also sees the IP header being added to the datagram.
• The physical layer is responsible for the delivery of signals from the source to the destination over a physical communication platform, which in this case is the Ethernet, and the header describing the Ethernet Protocol information is added to the Data Unit.
  

Putting it All Together - The Data Encapsulation Process

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• 1. One computer requests to send data to another over a network.
  
• 2. The data message flows through the Application Layer by using a TCP or UDP port to pass onto the internet layer.
• 3. The data segment obtains logical addressing at the Internet Layer via the IP protocol, and the data is then encapsulated into a datagram.
• 4. The datagram enters the Network Access Layer, where software will interface with the physical network. A data frame encapsulates the datagram for entry onto the physical network. At the end of the process, the frame is converted to a stream of bits that is then transmitted to the receiving computer.
• 5. The receiving computer removes the frame, and passes the packet onto the Internet Layer. The Internet Layer will then remove the header information and send the data to the Transport layer. Likewise, the Transport layer removes header information and passes data to the final layer. At this final layer the data is whole again, and can be read by the receiving computer if no errors are present.

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And there you have it: encapsulation at its finest.