Suggest an editImprove this articleRefine the answer for “What is the transport layer of the OSI model?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**The transport layer (Transport Layer, L4)** is the **fourth layer of the OSI model**, responsible for **delivering data from application to application** between devices on a network. The transport layer provides data transfer between programs on different devices. **Key point:** the transport layer uses ports, provides data segmentation, reliability (in TCP), and flow control.Shown above the full answer for quick recall.Answer (EN)Image**The transport layer (Transport Layer, L4)** is the **fourth layer of the OSI model**, responsible for **delivering data from application to application** between devices on a network. In short: **the transport layer provides data transfer between programs on different devices**. --- ## Why the transport layer is needed The network layer (L3) can deliver packets **from node to node**, but it **does not know** which application on the computer needs that data. The transport layer solves exactly this: - links **specific applications** - manages **reliability and order of delivery** - controls the **data flow** --- ## Main tasks of the transport layer ### 1. Delivering data "from application to application" The transport layer uses **ports**: - source port - destination port Thanks to ports: - one computer can simultaneously - work with many network programs Examples of ports: - 80 - HTTP - 443 - HTTPS - 22 - SSH --- ### 2. Data segmentation The transport layer: - splits application data into **segments** - sends them over the network - **reassembles** them on the receiver's side --- ### 3. Reliable delivery (when TCP is used) Depending on the protocol, the transport layer can: - acknowledge receipt of data - retransmit lost segments - ensure correct delivery order --- ### 4. Flow control The transport layer makes sure: - the sender does not transmit data faster - than the receiver can process it --- ### 5. Network congestion control Some protocols (TCP): - reduce transmission speed - if the network is congested --- ## Main transport layer protocols ### TCP (Transmission Control Protocol) - reliable - connection-oriented - guarantees delivery and order Used for: - websites - email - file transfers --- ### UDP (User Datagram Protocol) - unreliable - connectionless - faster and simpler Used for: - video and audio - online games - DNS --- ## 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 MAC addresses - does not transmit data as a signal --- ## Relationship to other layers - **L3 (network)** - delivers packets between nodes - **L4 (transport)** - delivers data between applications - **L7 (application)** - works with user data --- ## In short, for the interview > **The transport layer of the OSI model** is responsible for transferring data between applications on different devices, using ports, providing segmentation, reliability (TCP), and data flow control.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.