Transport Layer
Introduction to the Transport Layer
The Transport layer sits between the Network layer and the Application layer, providing end-to-end communication services for applications. It is responsible for delivering data between applications running on different hosts, handling issues like segmentation, flow control, and error recovery.
Core Functions of the Transport Layer
Segmentation and Reassembly
Breaking large messages into smaller segments for transmission and reassembling them at the destination.
Example: A 10MB file split into thousands of smaller segments for transmission
Connection Management
Establishing, maintaining, and terminating connections between applications.
Connection-oriented (TCP): Establishes a session before data transfer
Connectionless (UDP): Sends data without prior arrangement
Flow Control
Regulating the rate of data transmission to prevent overwhelming receivers.
Techniques: Sliding window protocols, rate-based mechanisms
Error Control
Detecting and recovering from transmission errors.
Methods: Checksums, acknowledgments, timeouts, retransmissions
Multiplexing and Demultiplexing
Managing multiple data streams from various applications.
Using port numbers to direct data to correct applications.
Example: Web browsing on port 80/443 while checking email on port 110/143
TCP vs UDP: The Primary Transport Protocols
TCP (Transmission Control Protocol)
TCP provides reliable, ordered, and error-checked delivery of data, making it suitable for applications that require high reliability but can tolerate some delay.
Key Features:
- Connection-oriented communication
- Three-way handshake for connection establishment
- Guaranteed delivery with acknowledgments
- In-order delivery of data
- Flow control and congestion control
Use Cases:
- Web browsing (HTTP/HTTPS)
- Email (SMTP, IMAP, POP3)
- File transfers (FTP, SFTP)
- Remote terminal access (SSH)
UDP (User Datagram Protocol)
UDP provides a simpler, connectionless service with minimal overhead, making it ideal for applications that prioritize speed over reliability.
Key Features:
- Connectionless communication
- No handshaking or connection establishment
- No guarantee of delivery or ordering
- Lower latency than TCP
- Simpler header structure (8 bytes vs. 20+ bytes for TCP)
Use Cases:
- Live streaming (video/audio)
- Online gaming
- Voice over IP (VoIP)
- DNS queries
- DHCP
Port Numbers: Addressing Applications
Transport layer protocols use port numbers to identify specific applications or services:
| Category | Range | Examples |
|---|---|---|
| Well-known ports | 0-1023 | HTTP: 80 HTTPS: 443 FTP: 20, 21 SSH: 22 |
| Registered ports | 1024-49151 | MySQL: 3306 PostgreSQL: 5432 RDP: 3389 |
| Dynamic/Private ports | 49152-65535 | Used for temporary connections |
Advanced Transport Layer Concepts
Congestion Control
TCP implements several algorithms to prevent network congestion:
- Slow Start: Begin with a small congestion window and double it with each acknowledgment
- Congestion Avoidance: Linear increase in window size after reaching a threshold
- Fast Retransmit: Resend packets when multiple duplicate ACKs are received
- Fast Recovery: Avoid slow start after fast retransmit
Quality of Service (QoS)
Transport protocols can support different levels of service quality:
- Bandwidth guarantees: Ensuring minimum throughput
- Latency bounds: Limiting maximum delay
- Jitter control: Reducing variation in packet arrival times
Transport Layer Security (TLS)
While technically in the application layer in the TCP/IP model, TLS provides secure communication services just above the transport layer:
- Encryption of data to prevent eavesdropping
- Authentication of endpoints to verify identity
- Message integrity checking to detect tampering
Interactive Transport Layer Visualization
Summary
The Transport layer bridges the gap between low-level network operations and high-level applications. By offering services like connection management, error control, and flow control, it enables reliable and efficient communication between applications across the network. The choice between TCP and UDP represents a fundamental tradeoff between reliability and speed that shapes how network applications function.