Intro — what this is and why it matters
The Transport Layer is the fourth layer of the OSI model (and the layer just above the Internet/Network layer in the TCP/IP model). Its job is process-to-process (end-to-end) delivery of data — it takes data from an application on one host and delivers it to the correct application on another host, across an unreliable network. The two dominant transport-layer protocols are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
For BCS, Bank IT, and IT-officer exams this is a guaranteed scoring topic: questions on "which is connection-oriented", "TCP vs UDP header size", "port ranges", "3-way handshake", "which protocol uses UDP" appear almost every year. Master this lesson and you can answer any objective or short-question on the transport layer.
Learning objectives
- Define the transport layer and explain its role of process-to-process delivery.
- Explain multiplexing/demultiplexing and the role of port numbers and sockets.
- Describe TCP: connection-oriented, reliable, byte-stream service; its header, flags, and the 3-way handshake and connection termination.
- Explain flow control, error control, and congestion control (slow start, congestion avoidance, fast retransmit).
- Describe UDP: connectionless, unreliable, message-oriented service; its 8-byte header.
- Compare TCP and UDP and decide which application uses which.
- Memorize the standard port numbers used by common protocols.
- Solve numerical problems: throughput, number of segments, sequence numbers, and bandwidth-delay product.