Transport Layer: TCP vs UDP, Ports, Segments, Reliability & Flow Control

TAKEAWAYS:

• TCP (Transmission Control Protocol) is connection-oriented, reliable, and uses three-way handshake for setup, while UDP (User Datagram Protocol) is connectionless, faster, and unreliable.
• TCP segments include sequence numbers, acknowledgments, checksums, and flags for reliability, while UDP has only source/destination ports, length, and checksum.
• Flow control (sliding window) and congestion control (slow start, AIMD) prevent network overload in TCP.
• Port numbers (0–65535) identify applications: well-known (0–1023), registered (1024–49151), and dynamic (49152–65535).
• TCP guarantees ordered, error-free delivery (used in HTTP, FTP, SSH), while UDP prioritizes speed (used in DNS, VoIP, video streaming).
• Real-world impact: eSewa uses TCP for secure transactions, WhatsApp uses UDP for low-latency messaging, and YouTube uses UDP for live streaming.

1. Introduction to the Transport Layer

The Transport Layer (Layer 4 of the OSI model) sits between the Network Layer (IP) and the Application Layer. Its primary role is to:

• Provide end-to-end communication between applications.
• Ensure reliable data transfer (TCP) or fast delivery (UDP).
• Multiplex/demultiplex data using port numbers.
• Handle flow control and congestion control.

Key Functions of the Transport Layer

classDiagram
    class TransportLayer {
        + Segment data into packets
        + Multiplex/demultiplex using ports
        + Ensure reliability (TCP) or speed (UDP)
        + Flow & congestion control
        + Error recovery (TCP only)
    }
    class TCP {
        + Connection-oriented
        + Reliable delivery
        + Three-way handshake
        + Flow control (sliding window)
        + Congestion control (AIMD)
    }
    class UDP {
        + Connectionless
        + No reliability guarantees
        + Faster delivery
        + No handshake
    }
    TransportLayer <|-- TCP
    TransportLayer <|-- UDP

2. TCP (Transmission Control Protocol)

TCP is a connection-oriented, reliable protocol used for applications requiring error-free data transfer.

TCP Segment Format

Key Fields Explained:

• Sequence Number: Identifies the order of segments.
• Acknowledgment Number: Confirms received data.
• Flags:
  • SYN: Synchronize (used in handshake).
  • ACK: Acknowledgment.
  • FIN: Terminate connection.
  • RST: Reset connection.
• Checksum: Detects errors in the segment.

TCP Three-Way Handshake

sequenceDiagram
    participant Client
    participant Server
    Client->>Server: SYN (Sequence Number = x)
    Server->>Client: SYN-ACK (Sequence Number = y, ACK = x+1)
    Client->>Server: ACK (ACK = y+1)
    Note over Client,Server: Connection Established

Example in Real Life: When you log into eSewa, TCP ensures your transaction details reach the server without errors via this handshake before data transfer begins.

3. UDP (User Datagram Protocol)

UDP is a connectionless, unreliable protocol used for speed-critical applications.

UDP Segment Format

Key Differences from TCP:

Why Use UDP?

• Low latency (critical for live video streaming like YouTube).
• No connection setup (faster for DNS queries).
• Lightweight (used in online gaming like PUBG).

Example in Real Life: When you watch a YouTube live stream, UDP sends video packets without waiting for acknowledgments, reducing delay.

4. Port Numbers and Multiplexing

Ports (0–65535) identify applications on a device.

Example in Real Life: When you visit Daraz, your browser uses port 80 (HTTP) or 443 (HTTPS) to communicate with Daraz’s server.

5. Flow Control and Congestion Control in TCP

Flow Control (Sliding Window)

• Prevents the sender from overwhelming the receiver.
• Uses window size to adjust data transmission rate.

stateDiagram-v2
    [*] --> Idle
    Idle --> Sending: Data sent
    Sending --> Waiting: ACK received
    Waiting --> Sending: Window updated
    Sending --> [*]: All data sent

Congestion Control (AIMD Algorithm)

• Additive Increase/Multiplicative Decrease (AIMD):
  • Increase window size exponentially (slow start).
  • Decrease window size multiplicatively (congestion detected).

Example in Real Life: When Ncell’s 4G network is congested, TCP reduces data speed to avoid packet loss.

6. TCP vs. UDP: Comparison Table

7. Real-World Applications

1. eSewa (TCP)

• Uses TCP for secure financial transactions.
• Ensures no data loss during payment processing.

2. WhatsApp (UDP for Messaging, TCP for File Transfer)

• Uses UDP for instant messaging (low latency).
• Uses TCP for file sharing (reliable transfer).

3. YouTube Live Streaming (UDP)

• Uses UDP to minimize delay in live broadcasts.

4. NTC’s Internet Service (TCP for Web Browsing)

• When you browse ntc.net.np, TCP ensures all web pages load correctly.

5. Pathao’s Ride Booking (UDP for GPS Updates)

• Uses UDP to send real-time GPS location of drivers.

8. Worked Example: TCP Connection Setup (eSewa Login)

Scenario: You log into eSewa to pay a bill.

1. Client (Your Phone) → Server (eSewa): SYN (Sequence Number = 1000)
2. Server → Client: SYN-ACK (Sequence Number = 2000, ACK = 1001)
3. Client → Server: ACK (ACK = 2001)
4. Connection Established!
   • Now, TCP ensures all transaction data is delivered correctly.

9. Worked Example: UDP in Video Streaming (YouTube)

Scenario: You watch a YouTube live stream.

• No handshake (UDP is connectionless).
• Packets may arrive out of order or lost (but playback continues smoothly).
• Checksum detects errors but does not retransmit (unlike TCP).

10. Exam Tip

How to Score Full Marks in Exams: ✅ Compare TCP and UDP in a table (as above). ✅ Draw the TCP segment format and explain each field. ✅ Explain the three-way handshake with a sequence diagram. ✅ Relate real-world apps (eSewa, WhatsApp, YouTube) to TCP/UDP. ✅ Describe flow control and congestion control with sliding window and AIMD. ✅ Mention port numbers (well-known, registered, dynamic).

Common Mistakes to Avoid: ❌ Forgetting SYN, ACK, FIN flags in TCP. ❌ Confusing UDP’s speed with TCP’s reliability. ❌ Not explaining why UDP is used in VoIP/DNS.