Transport Layer

Introduction

• The transport layer is the 4th layer of the OSI model.
• This layer lies in between the Network layer and the Session layer.

Features

• This layer acts as an interface between the bottom and top three layers.
• The lower data link layer (layer 2) is only responsible for delivering data packets from one node to another in the LAN area whereas in WAN it is done by the transport layer. Thus, if a data packet gets lost in a router somewhere in the WAN area, the transport layer will detect that.
• This layer determines how much data should be sent where and at what rate. 

Functions

• End-to-End/Port-to-Port Data Delivery:
  • Also called Process-to-Process delivery.
• Reliability:
  • In this layer, data is delivered to the receiver completely(no loss of data – all the data packets) and in the same sequence as sent by the sender(in order way) using TCP. 
  • The transport layer is responsible for complete end-to-end validity and integrity of the data transmission i.e. it ensures that data is successfully sent and received between two computers in a network. For example – it ensures that if a 5MB file is sent, the full 5MB is received.
  • The transport layer provides a network-independent, reliable message-interchange service to the upper-top application-oriented layers.
  • The transport layer provides a reliable message transfer service by hiding the detailed operation of the underlying network from the session layer i.e. transport layer makes sure that the message is delivered to the correct process(port no.) on the destination machine.
  • The transport layer also helps to control the reliability of a link between sender and receiver through flow control, error control, segmentation, or desegmentation.
• Segmentation:
  • The transport layer splits the continuous stream of data/messages, obtained from the upper/session layer, into numerous segments and numbers them sequentially for efficient and effective transmission at the sender’s end while desegmentation at the receiver’s end.  
• Congestion/Flow Control:
  • Same as Data Link Layer but in WAN only.
• Error Control:
  • The transport layer ensures data transmission (in WAN) without any error i.e. if data is sent incorrectly, this layer has the responsibility of asking for retransmission of the data using retransmission technology.
  • Error control is done at the receiver end using the Checksum method mainly.
• Multiplexing & Demultiplexing:
  • This layer may send multiple applications data through the single channel at the sender’s end using multiplexing techniques whereas receives and separates those data at the receiver’s end using demultiplexing techniques.