Transport Layer

The transport layer supports communication between applications running on different devices. It segments data, identifies applications with port numbers, and provides either reliable or lightweight delivery depending on the protocol used.

The transport layer is responsible for process-to-process communication. While IP delivers packets between hosts, transport protocols deliver data to the correct application.

Responsibilities may include:

• Segmenting data
• Reassembling data
• Tracking conversations
• Identifying applications with port numbers
• Providing reliability when required
• Controlling flow between sender and receiver

The two major transport layer protocols are TCP and UDP.

TCP is used when reliability is more important. UDP is used when speed, simplicity, or low delay is more important.

TCP provides reliable delivery through sequencing, acknowledgments, retransmissions, and flow control. If data is lost, TCP can resend it. If data arrives out of order, TCP can reorder it before passing it to the application.

TCP begins communication with a three-way handshake.

1. SYN
2. SYN-ACK
3. ACK

This process establishes a session between the two endpoints.

When communication is finished, TCP closes the connection gracefully using control messages. This helps both sides know that no more data will be sent.

UDP is simpler than TCP. It does not establish a session, does not track acknowledgments, and does not retransmit lost data. Applications that use UDP may handle reliability themselves or may tolerate some loss.

UDP is useful for real-time traffic where waiting for retransmission may be worse than losing a small amount of data.

Port numbers identify applications and services.

Common examples:

Applications choose TCP or UDP based on their needs. A file download should be complete and accurate, so TCP is appropriate. A live voice call needs low delay, so UDP is often preferred.