TCP/IP Model vs OSI Model

We have talked in other posts about the OSI model, which is the de-facto standard model for describing and implementing IP communication networks. You might be thinking that the OSI model was the original standard created when the first computer networks started to appear, but this is wrong. The TCP/IP model is older than the OSI, despite that the OSI is the most well known model today.

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OSI Layer 7: Application Layer

The Application Layer is the one which interacts with the user. A Software application that implements a communicating component, uses the OSI Application Layer to establish the application’s communication. For example a word processor that does not have communications capabilities, would not be concerned with the OSI Application Layer. On the other hand, an application with communication capabilities (e.g a Web Browser), has to implement the OSI Application Layer standards.
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OSI Layer 5: Session Layer

The Session Layer starts, controls, and manages communication conversations (sessions). Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. The session layer creates ways to imply which flows are part of the same session and which flows must complete before any are considered complete. Continue reading “OSI Layer 5: Session Layer”

OSI Layer 4: Transport Layer

The Transport Layer (e.g TCP) provides reliable delivery of Data by using error checking, acknowledgments, flow control, and sequence checking. Multiplexing of incoming data for different flows to applications on the same host (for example, TCP ports) is also performed. Flow control in this layer ensures that the transmitting device does not send more data than the receiving device can process.

Examples of Layer 4 Specifications and Protocols
• TCP
• UDP
• SPX
• Port Numbers (e.g 80 for HTTP)

OSI Layer 3: Network Layer

This Layer defines logical addressing (e.g IP address) in order to identify each endpoint in a network and provide end-to-end delivery of packets. Think of the Network Layer as the postal service in a country. When you want to send a letter to a friend, you just need to know the destination postal address (e.g destination IP address) and you send the letter. The postal service (e.g the Network Layer) will determine the path to the destination (e.g IP Path Determination) and take care of the delivery (e.g using IP Routing). The two endpoints do not need to know the exact path taken for the delivery of the letter.

This Layer also encapsulates data into Packets, and defines how to fragment a packet into smaller pieces to accommodate media with smaller Maximum Transmission Units. An IP Router works in this Layer, since it can use the IP address information to route packets.

Examples of Layer 3 Specifications and Protocols
• IP
• IPX
• AppleTalk