What tasks does the transport layer solve?

The transport layer (L4) solves tasks related to transferring data between applications on different devices and controlling that transfer.

In simpler terms: the transport layer makes sure data reaches from one program to another correctly and on time.

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#Main tasks of the transport layer

#1. Delivering data from application to application

The transport layer uses ports:

• source port
• destination port

This allows:

• one computer to simultaneously
• serve many network programs

Example:

• browser → port 443
• email client → port 25

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#2. Segmentation and reassembly of data

The transport layer:

• splits large application data into segments
• sends them over the network
• reassembles them into the original stream on the receiver's side

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#3. Ensuring reliability (TCP)

If TCP is used, the transport layer:

• acknowledges receipt of data
• retransmits lost segments
• ensures correct delivery order

UDP does not do this.

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#4. Managing delivery order

The transport layer:

• numbers segments
• assembles them in the correct sequence
• protects against "mixed-up" data

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#5. Flow control

The transport layer makes sure that:

• the sender does not "overwhelm" the receiver with data
• the transmission speed matches the receiving capacity

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#6. Network congestion control

(mostly TCP)

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

This helps the network run more stably.

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#What the transport layer does NOT do

Important for the interview. The transport layer:

• does not choose the route (that is L3)
• does not work with IP or MAC addresses
• is not responsible for delivering signals

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#Brief comparison of TCP and UDP

• TCP - reliability, order, control
• UDP - speed, simplicity, minimal checks

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#In short, for the interview

The transport layer handles delivering data between applications, segmenting and reassembling data, managing ports, ensuring reliability, and controlling order, flow, and congestion (depending on the protocol).