Transport Protocols

UDP — User Datagram Protocol

UDP is a transport layer protocol that operates on top of IP.

Structure of a UDP Packet

┌──────────┬────────────────────┬──────────────┐
│ IP Head  │  Port + Checksum   │ Data Payload │
│          │   (UDP header)     │              │
└──────────┴────────────────────┴──────────────┘

Two Main Features UDP Adds to IP

1. Port Numbers — Identify which application/program the packet is intended for on the destination device.

2. Checksum — Detects data corruption:

• Sender runs a checksum algorithm on the data payload and inserts the result into the UDP header.
• Receiver runs the same algorithm and compares. If they match → data is intact. If not → data is corrupt.

What UDP Does NOT Have

• No acknowledgement that data arrived
• No re-sending of lost packets
• No guaranteed ordering

When to Use UDP

UDP is used when speed matters more than reliability:

TCP — Transmission Control Protocol

TCP solves the problems UDP ignores. It adds reliability on top of IP.

Why TCP Exists

Because the internet routes packets independently:

• Packets can arrive out of order
• Packets can be lost or corrupted
• UDP has no way to handle these situations

Two Major Features of TCP

1. Acknowledgement (ACK)

• When a TCP packet arrives at the destination, the receiver sends back an ACK (acknowledgement) to the sender.
• If the sender doesn’t receive an ACK within a timeout window, it re-sends the packet.
• This guarantees delivery and fixes lost/corrupted packets.

2. Sequential Ordering

• Every TCP packet has a sequence number.
• Even if packets arrive out of order, the receiver reassembles them in the correct order using sequence numbers.

TCP 3-Way Handshake (Connection Setup)

Before any data is sent, TCP establishes a connection:

   Client                     Server
     │                           │
     │──────── SYN ────────────→ │  "I want to connect"
     │                           │
     │ ←──── SYN + ACK ──────── │  "OK, I'm ready"
     │                           │
     │──────── ACK ────────────→ │  "Great, let's go"
     │                           │
     │   ←── Data flows ──→      │

TCP 4-Way Handshake (Connection Teardown)

   Client                     Server
     │──────── FIN ────────────→ │
     │ ←──────── ACK ─────────── │
     │ ←──────── FIN ─────────── │
     │──────── ACK ────────────→ │

TCP vs. UDP Comparison