Transport Layer Functions and TCP Flow Control Explained

The Transport Layer: OSI Model Layer 4

The Transport Layer is the 4th layer of the OSI model, providing process-to-process communication between applications running on different hosts. It ensures reliable, error-free, and ordered delivery of data.

Main Services of the Transport Layer

• Process-to-Process Delivery: Uses port numbers to deliver data from a specific process on the sender to a specific process on the receiver.
• Segmentation and Reassembly: Divides large messages into smaller segments for transmission and reassembles them at the destination.
• Connection Control: Supports connection-oriented communication (TCP) and connectionless communication (UDP).
• Flow Control: Prevents the sender from overwhelming the receiver with too much data by using techniques like the sliding window.
• Error Control: Uses acknowledgments, retransmissions, and checksums to detect and correct errors.
• Congestion Control: Avoids network overload by adjusting the rate of data transmission. TCP uses slow start, congestion avoidance, fast retransmit, and fast recovery.
• Multiplexing and Demultiplexing: Allows multiple applications to share a single network connection and distributes data to the correct processes.

TCP Flow Control Mechanism

TCP Flow Control is a mechanism used to ensure that a sender does not overwhelm the receiver by sending data faster than it can process. It prevents buffer overflow and ensures smooth, reliable data transfer. Flow control is implemented using the Sliding Window Protocol in TCP.

TCP Sliding Window Mechanism

TCP uses a receiver window (rwnd) to indicate how much data the receiver can accept. This window size is included in every ACK message sent by the receiver. The sender can transmit only that amount of unacknowledged data equal to the window size.

The sliding window adjusts dynamically depending on receiver capacity:

• If the receiver’s buffer has space, the window size increases.
• If the receiver is overloaded, it reduces the window size to slow down the sender.
• If the window size becomes zero, the sender must stop sending until it receives an update.

Example: If rwnd = 5000 bytes, the sender can send 5000 bytes before waiting for acknowledgment.