Middle
What happens at the transport layer?
sobes.tech AI
Answer from AI
At the Transport Layer of the OSI/TCP/IP model, interaction occurs between applications on different hosts, not between the hosts themselves. Main functions:
- Segmentation and reassembly of data: Data from upper layers is divided into segments (for TCP) or datagrams (for UDP) and passed to the network layer. On the receiving side, these parts are reassembled.
- Application identification (Ports): Port numbers are used to distinguish applications running on the same host. For example, a web server typically uses port 80 (HTTP) or 443 (HTTPS).
- Flow control (for TCP): Manages the data transfer rate between sender and receiver to prevent buffer overflow.
- Error control (for TCP): Ensures reliable data delivery using sequence numbers, acknowledgments (ACKs), and retransmission of lost segments. UDP does not have built-in error control at this level.
- Connection management (for TCP): Establishes, maintains, and terminates a logical connection between applications. A typical example is the "three-way handshake" for establishing a TCP connection. UDP is a connectionless protocol.
Two main transport layer protocols:
- TCP (Transmission Control Protocol): Reliable, connection-oriented protocol. Guarantees data delivery in the correct order. Used for applications requiring high reliability (e.g., HTTP, FTP, SMTP).
- UDP (User Datagram Protocol): Unreliable, connectionless protocol. Does not guarantee delivery or order of packets. Used for applications where speed is more important than reliability (e.g., DNS, DHCP, streaming video/audio).
Comparison of TCP and UDP:
| Characteristic | TCP | UDP |
|---|---|---|
| Type | Connection-oriented | Connectionless |
| Reliability | High (guaranteed delivery) | Low (no delivery guarantees) |
| Error control | Present (ACK, retransmission) | Absent |
| Flow control | Present | Absent |
| Delivery order | Guaranteed | Not guaranteed |
| Overhead | High | Low |
| Speed | Lower | Higher |
| Application examples | HTTP, FTP, SMTP, SSH | DNS, DHCP, VoIP, streaming video/audio |
At this level, logical interaction occurs between processes running on end hosts, providing services to the network layer (transmitting segments/datagrams) and receiving data from the session layer or application layer, depending on the model.