Transportlayer

The transport layer is the fourth layer of the OSI model and a fundamental part of the Internet protocol suite. It provides end-to-end communication services for applications running on host machines. Positioned above the network layer (IP) and below the application layer in common model mappings, the transport layer delivers data between processes identified by port numbers, enabling multiplexing of multiple conversations over a single network connection.

Key functions include segmentation or message boundaries, reliable data transfer, flow control, congestion control, error detection, and connection management. It abstracts away differences in the underlying network to present a consistent communication service to applications. The layer is largely implemented by two main protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

TCP offers a reliable, connection-oriented service with in-order delivery, sequencing, acknowledgments, and flow and congestion control. It establishes a connection through a three-way handshake and provides a continuous byte stream suitable for many applications such as web browsing and email.

UDP provides a lightweight, connectionless, best-effort service with minimal overhead and no guarantee of delivery or ordering. It is well suited for real-time, streaming, or multicast applications where low latency is prioritized over reliability.

Other transport protocols exist, such as SCTP (which supports multi-streaming and multi-homing) and DCCP (datagram congestion-controlled). More recently, QUIC, built over UDP, provides reliable, multiplexed streams with strong encryption and aims to reduce latency.

In practice, applications select the transport protocol that matches their requirements for reliability, latency, and ordering.