TCP (Transmission Control Protocol)

The Reliable Protocol

TCP (Transmission Control Protocol) is the workhorse of the internet. It provides reliable, ordered, error-checked delivery of data. When you load a webpage, send an email, or transfer a file, TCP makes sure every byte arrives correctly.

TCP is like a certified mail service — every piece of data is tracked, acknowledged, and retransmitted if lost. It's slower than UDP, but you can trust it with your data.

The Three-Way Handshake

Before TCP can send data, it establishes a connection using the famous three-way handshake:

  Client                          Server
    │                               │
    │──── SYN ─────────────────────→│  Step 1: Client says "I want to connect"
    │                               │
    │←─── SYN-ACK ─────────────────│  Step 2: Server says "OK, I'm ready too"
    │                               │
    │──── ACK ─────────────────────→│  Step 3: Client says "Great, let's start"
    │                               │
    │═══════════════════════════════│
    │     Connection Established    │
    │═══════════════════════════════│

This handshake ensures both sides are ready and agree on initial sequence numbers. It's like two people confirming they're both on the phone before starting a conversation.

How TCP Ensures Reliability

• Sequence Numbers — Every byte of data is numbered. The receiver can detect missing data and reorder out-of-order segments.
• Acknowledgments (ACKs) — The receiver sends ACKs to confirm which data it has received. If the sender doesn't receive an ACK within a timeout period, it retransmits the data.
• Checksum — Each segment includes a checksum. If the data is corrupted in transit, the receiver detects it and discards the segment.
• Flow Control — The receiver tells the sender how much buffer space it has available (the window size). This prevents the sender from overwhelming the receiver.
• Congestion Control — TCP monitors network conditions and slows down when congestion is detected. Algorithms like Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery manage this.

TCP Segment Structure

  ┌────────────┬────────────┬──────────────────────────┐
  │ Src Port   │ Dst Port   │                          │
  │ (16 bits)  │ (16 bits)  │                          │
  ├────────────┼────────────┤      TCP Header          │
  │ Sequence Number          │      (20-60 bytes)      │
  │ (32 bits)                │                          │
  ├──────────────────────────┤                          │
  │ Acknowledgment Number    │                          │
  │ (32 bits)                │                          │
  ├───────┬─────┬────────────┤                          │
  │Header │Flags│  Window    │                          │
  │Length │SYN  │  Size      │                          │
  │       │ACK  │  (16 bits) │                          │
  │       │FIN  │            │                          │
  │       │RST  │            │                          │
  ├────────────┴────────────┼──────────────────────────┤
  │ Checksum    │ Urgent Ptr│        Data              │
  └────────────┴────────────┴──────────────────────────┘

TCP Connection Termination

When a TCP connection is done, it's closed with a four-way handshake:

  Client                          Server
    │                               │
    │──── FIN ─────────────────────→│  "I'm done sending"
    │←─── ACK ─────────────────────│  "Acknowledged"
    │←─── FIN ─────────────────────│  "I'm done too"
    │──── ACK ─────────────────────→│  "Acknowledged"
    │                               │
    │     Connection Closed         │

Both sides must independently close their sending direction. This ensures all data is delivered before the connection is fully terminated.