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Layer 4. Transport Layer

The fourth layer of the seven layers of Open Systems Interconnection (OSI) network mode is the Transport layer. The Transport layer handles transport functions such as reliable or unreliable delivery of the data to the destination. On the sending computer, the transport layer is responsible for breaking the data into smaller packets, so that if any packet is lost during transmission, the missing packets will be sent again. Missing packets are determined by acknowledgments (ACKs) from the remote device, when the remote device receives the packets. At the receiving system, the transport layer will be responsible for opening all of the packets and reconstructing the original message.

Another function of the transport layer is TCP segment sequencing. Sequencing is a connection-oriented service that takes TCP segments that are received out of order and place them in the right order.

The transport layer also enables the option of specifying a "service address" for the services or application on the source and the destination computer to specify what application the request came from and what application the request is going to.

Many network applications can run on a computer simultaneously and there should be some mechanism to identify which application should receive the incoming data. To make this work correctly, incoming data from different applications are multiplexed at the Transport layer and sent to the bottom layers. On the other side of the communication, the data received from the bottom layers are de-multiplexed at the Transport layer and delivered to the correct application. This is achieved by using "Port Numbers".

The protocols operating at the Transport Layer, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) uses a mechanism known as "Port Number" to enable multiplexing and de-multiplexing. Port numbers identify the originating network application on the source computer and destination network application on the receiving computer.

Layer 5. Session Layer
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