The four main functions of the Transport layer are:

Segment and assemble upper-layer applications

Transport segments from one host to another host

Establish and manage end-to-end operations

Error recovery

In carrying out its duties, the Transport layer performs a range of support activities, including:

Maintaining data integrity through flow control techniques

Multiplexing the data from upper layer applications

Setting up and tearing down any virtual circuits established to transport the data over the network

Hiding any network-dependent information from the upper layers (which will only confuse them)

Breaking down Session layer (layer 5) datagrams into segments

Monitoring the error-free delivery of the data to its destination

Providing for general connection management and data transfer services

Providing for the reliable (but, not guaranteed) delivery of data

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