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Transport Layer

Transport Layer(Layer 4): The transport layer is responsible for the delivery of an entire message from one process to another. It ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.
 Functions include:
  • Service-point Addressing: Computers often run more than one program at the same time. For this reason, source-to-destination delivery means delivery not only from one computer(source) to another(receiver), but also from a specific process on one computer to a specific process on the other. The transport layer header, therefore, must include a type of address, called service point address(port address). The Network layer is responsible for sending each packet to the correct computer; the transport layer gets the message to the correct process on that computer.
  • Segmentation and Reassembly: A message is divided into transmittable segments, each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly at the destination and to identify and replace packets, that were lost in the transmission.
  • Connection Control: This layer can be connection-less or connection-oriented. A connection-oriented transport layer makes a connection with the transport layer at the destination machine first, before packet delivery. The connection is terminated after all the data is sent. A connection-less transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine.
  • Flow and Error Control: Unlike data link layer, both error control as well as flow control at this layer, is performed end-to-end or process-to-process, rather than across a single link.
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