## TCP/IP Protocol Suite

The ISO model is theoretical. TCP/IP is what we actually use.

### TCP (Transmission Control Protocol)
**Connection-Oriented**. "Reliable".
1.  **The 3-Way Handshake**:
    *   **SYN** (Client): "Hi, can we talk?"
    *   **SYN-ACK** (Server): "Yes, I am open."
    *   **ACK** (Client): "Great, here is data."
2.  **Guarantees Delivery**: If a packet is lost, TCP resends it.
3.  **Use Case**: Web browsing (HTTP), Email (SMTP), File Transfer (FTP). You don't want a missing pixel in your file.

### UDP (User Datagram Protocol)
**Connection-less**. "Fire and Forget".
1.  No Handshake. No Guarantee.
2.  The sender just blasts data. If you miss it, tough luck.
3.  **Use Case**: Streaming Video, VoIP (Voice), DNS.
    *   *Why?* Speed. If you drop a frame in a video, you don't want to pause and wait for it to resend. You just move to the next frame.

### Headers
*   **TCP Header**: Source Port, Dest Port, Sequence Number, Flags (SYN/ACK/FIN/RST).
*   **IP Header**: Source IP, Dest IP, TTL (Time To Live).