What is the transport layer responsible for in an end-to-end connection?

In an end-to-end connection, the transport layer is responsible for the entire logic of transferring data between two end applications - from sending to correct receipt.

In simpler terms: the transport layer makes sure data reaches from one program to another correctly, in the right order, and at the right speed (when required).

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#What exactly the transport layer is responsible for (end-to-end)

#1. Identifying applications (ports)

The transport layer:

• uses ports
• precisely determines:
  • which application the data came from
  • which application it is intended for

Without this, end-to-end communication between programs would be impossible.

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#2. Establishing and closing the connection (TCP)

If TCP is used:

• a connection is established before the transfer
• it is closed correctly after the exchange ends

This happens between the end nodes, not inside the network.

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#3. Reliable delivery (TCP)

The transport layer provides:

• acknowledgment of received data
• retransmission of lost segments
• protection against duplicates

Important:

• all of this happens only between the sender and the receiver
• routers do not take part in this

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#4. Order of data delivery

The transport layer:

• numbers segments
• assembles them in the correct sequence
• passes data to the application already in the correct form

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#5. Flow control

The transport layer makes sure that:

• the sender does not transmit data faster
• than the receiver can accept it

This protects the receiver, not the network.

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#6. Network congestion control (TCP)

The transport layer:

• reduces transmission speed when congested
• increases it when the network is free

Decisions are made:

• based on end-to-end observations
• without the involvement of intermediate nodes

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#7. Segmentation and reassembly of data

The transport layer:

• splits application data into segments
• reassembles them on the receiver's side

Applications receive:

• a complete data stream
• without needing to know how it was transmitted

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#What the transport layer does NOT do in end-to-end

Important for the interview. The transport layer:

• does not choose the route (that is L3)
• does not manage the transfer at every network segment
• does not work with MAC addresses
• does not guarantee physical delivery of signals

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#The key idea of end-to-end

• transfer control is at the ends
• the network inside is as simple as possible
• reliability and order are the job of L4