Transport Layer: TCP vs UDP, Segment Headers, Reliability & Flow Control

TAKEAWAYS:

• TCP guarantees reliable, ordered delivery via acknowledgments, retransmissions, and sequence numbers, while UDP offers speed over accuracy with no handshake or error recovery.
• The TCP segment header (20–60 bytes) includes flags (SYN, ACK, FIN), sequence/acknowledgment numbers, and checksums; UDP’s header (8 bytes) only has source/destination ports and checksum.
• Flow control (TCP’s sliding window) prevents sender overload by dynamically adjusting window size, while congestion control (slow start, AIMD) manages network traffic.
• Port numbers (0–65535) identify applications: HTTP (80), HTTPS (443), DNS (53), and ephemeral ports (>1024) for client responses.
• Real-world trade-offs: WhatsApp uses UDP for voice calls (low latency), while eSewa uses TCP for transactions (no data loss).
• Exam focus: Compare TCP/UDP headers, explain three-way handshake, and calculate window sizes or sequence numbers in worked examples.

1. Transport Layer Overview: Why TCP and UDP?

The Transport Layer (Layer 4) sits between the Network Layer (IP) and Application Layer (HTTP, FTP, DNS). Its job:

• Segmentation: Breaks application data into smaller segments (TCP) or datagrams (UDP) for transmission.
• Multiplexing/Demultiplexing: Uses port numbers to direct data to the correct application.
• Reliability: Ensures data arrives correctly and in order (TCP) or delivers it as-is (UDP).

2. UDP: The "Fire-and-Forget" Protocol

UDP (User Datagram Protocol) is fast but unreliable. It’s used where speed matters more than accuracy.

Key Features

• No connection setup: Sends data immediately (no handshake).
• No retransmissions: If a packet is lost, the app must handle it (e.g., video glitches).
• No ordering guarantees: Packets may arrive out of order.
• Minimal overhead: Only 8-byte header (vs. TCP’s 20–60 bytes).

UDP Header Format

Why UDP?

• DNS queries (port 53): Speed > reliability.
• VoIP (WhatsApp calls): Dropped packets cause minor glitches, not crashes.
• Online gaming: Low latency is critical; lost packets are acceptable.

Worked Example: WhatsApp Voice Call

• WhatsApp uses UDP for voice calls because:
  • A 50ms delay in retransmitting a lost packet feels like a 10-second lag to the user.
  • 1–2% packet loss is tolerable (human ear doesn’t notice).
• If TCP were used, a single lost packet could stall the call until retransmitted.

3. TCP: The Reliable Workhorse

TCP (Transmission Control Protocol) ensures data arrives correctly, in order, and without loss. It’s used for web browsing, emails, file transfers.

Key Features

• Connection-oriented: Uses a three-way handshake to establish a connection.
• Reliable delivery: Retransmits lost packets and reorders out-of-sequence data.
• Flow control: Adjusts sending rate to match the receiver’s capacity.
• Congestion control: Avoids overwhelming the network.

TCP Three-Way Handshake

sequenceDiagram
    participant Client as Client (e.g., Browser)
    participant Server as Server (e.g., Google)
    Client->>Server: SYN (Seq=1000)
    Server->>Client: SYN-ACK (Seq=2000, Ack=1001)
    Client->>Server: ACK (Seq=1001, Ack=2001)
    Note over Client,Server: Connection Established!

Worked Example: eSewa Transaction

1. Your phone (client) sends a SYN to eSewa’s server.
2. Server replies SYN-ACK.
3. You send ACK → connection open.
4. You send payment data → server acknowledges receipt.
5. If a packet is lost, TCP retransmits it before closing the connection (FIN handshake).

TCP Segment Header

Critical Fields Explained

4. TCP Reliability Mechanisms

TCP ensures reliability through:

1. Acknowledgments (ACKs): Receiver sends ACKs for every segment.
2. Retransmissions: If ACK isn’t received within a timeout, TCP resends.
3. Sequence Numbers: Ensures packets are reassembled in order.
4. Checksum: Detects corrupted data (discards if invalid).

Worked Example: Downloading a File from Daraz

• You request a 10MB file → Daraz’s server sends it in TCP segments.
• If Segment 5 is lost:
  • Your device times out waiting for ACK.
  • Daraz retransmits Segment 5.
  • You ACK Segment 5 → download resumes.

5. Flow Control: Sliding Window

TCP uses a sliding window to control data flow and prevent overload.

How It Works

1. Sender sends up to Window Size bytes (e.g., 1000 bytes).
2. Receiver sends ACK → window "slides" forward.
3. If receiver is slow, it reduces window size (e.g., to 500 bytes).

Worked Example: Kathmandu Traffic (Analogy)

• Imagine a one-lane road (window size = 1 car at a time).
• If traffic ahead is slow (small window), you reduce speed (send fewer packets).
• If traffic clears (large window), you speed up (send more packets).

6. Congestion Control: Avoiding Network Collapse

TCP monitors network congestion and adjusts sending rate using:

• Slow Start: Exponentially increases window size until loss occurs.
• AIMD (Additive Increase, Multiplicative Decrease): Gradually increases window, then halves it after packet loss.

Real-World Impact

• Without congestion control, YouTube videos would buffer constantly during peak hours.
• NTC’s fiber network uses TCP’s AIMD to avoid overloading during exams.

7. TCP vs. UDP: Comparison Table

8. Port Numbers: The Application’s Address

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

• Well-known ports (0–1023): Assigned by IANA (e.g., 80 = HTTP).
• Registered ports (1024–49151): Used by apps (e.g., 5432 = PostgreSQL).
• Ephemeral ports (49152–65535): Temporary client ports (e.g., 54321).

Example: Accessing a Website

1. Your browser opens an ephemeral port (e.g., 54321).
2. It sends a request to port 80 (HTTP) or 443 (HTTPS) on the server.
3. Server replies on port 80/443 → your browser reads it via 54321.

9. In the Real World

1. WhatsApp (UDP for Voice Calls)

• Why UDP?
  • Voice packets must arrive within 30ms to sound natural.
  • 1–2% packet loss is acceptable (human ear fills gaps).
  • TCP’s retransmissions would cause unacceptable delays.

2. eSewa (TCP for Transactions)

• Why TCP?
  • No lost packets: A missing "transfer money" packet means failed transaction.
  • Ordered delivery: Ensures account numbers and amounts arrive correctly.
  • Flow control: Prevents server overload during peak hours (e.g., Dashain).

3. YouTube (UDP + TCP Hybrid)

• Video streaming uses UDP for most packets (speed > perfection).
• Critical metadata (e.g., subtitles) uses TCP to ensure accuracy.

4. NTC’s Fiber Network (TCP Congestion Control)

• During exam seasons, NTC’s routers use TCP AIMD to:
  • Detect congestion (packet loss).
  • Reduce bandwidth for non-urgent traffic (e.g., Netflix).
  • Prioritize exams (lower latency for TU/PU portals).

5. Online Gaming (UDP + Custom Protocols)

• Games like PUBG use UDP for gameplay but add:
  • Client-side prediction: Assumes actions happened (even if packets are lost).
  • Server reconciliation: Fixes mistakes after the fact.

10. Exam Tip: How to Score Full Marks

Common Exam Questions & How to Answer

Tips for Worked Examples

1. Sequence Numbers: Always show Seq and Ack values (e.g., "Seq=1000, Ack=2001").
2. Window Size: Calculate based on bytes sent and ACK received (e.g., "Window slides from 1000–2000 after ACK").
3. Retransmissions: State timeout and retransmit logic (e.g., "No ACK for Segment 5 → retransmit").
4. Real-World Tie-Ins: Link answers to eSewa (TCP), WhatsApp (UDP), or NTC congestion.

Avoid These Mistakes

• ❌ Forgetting flags in TCP header (SYN, ACK, FIN).
• ❌ Mixing up sequence number (sender’s byte count) and acknowledgment number (next expected byte).
• ❌ Ignoring port numbers in UDP/TCP questions.
• ❌ Describing UDP as "unreliable" without explaining why it’s acceptable (e.g., VoIP).