The TCP/IP Stack

While the OSI model is a useful teaching framework, the TCP/IP model is what the internet actually runs on. Developed in the 1970s by Vint Cerf and Bob Kahn, it’s simpler, more pragmatic, and battle-tested by decades of real-world use.

Four Layers vs. Seven

The TCP/IP model condenses networking into four layers:

┌─────────────────────────────────────────────────────────────┐
│          TCP/IP Model          │       OSI Model           │
├─────────────────────────────────────────────────────────────┤
│                                │   Application  (Layer 7)  │
│   Application Layer            │   Presentation (Layer 6)  │
│                                │   Session      (Layer 5)  │
├─────────────────────────────────────────────────────────────┤
│   Transport Layer              │   Transport    (Layer 4)  │
├─────────────────────────────────────────────────────────────┤
│   Internet Layer               │   Network      (Layer 3)  │
├─────────────────────────────────────────────────────────────┤
│   Network Access Layer         │   Data Link    (Layer 2)  │
│   (Link Layer)                 │   Physical     (Layer 1)  │
└─────────────────────────────────────────────────────────────┘

This simplification isn’t accidental—it reflects reality. The top three OSI layers often blend together in practice, and the bottom two are typically handled by the same hardware/drivers.

Layer 1: Network Access Layer

Also called the Link Layer, this combines OSI’s physical and data link layers. It handles everything needed to send packets across a physical network segment.

Responsibilities:

• Physical transmission
• MAC addressing
• Frame formatting
• Local delivery

The TCP/IP model is agnostic about this layer. Whether you’re using:

• Ethernet
• WiFi
• Cellular (4G/5G)
• Satellite
• Carrier pigeon (yes, there’s an RFC for that: RFC 1149)

…the upper layers don’t care. This abstraction is what allows the internet to work across wildly different physical media.

Layer 2: Internet Layer

The internet layer handles logical addressing and routing. Its job is getting packets from source to destination across multiple networks.

Key Protocol: IP (Internet Protocol)

IP's Job: Get this packet from A to B, somehow.

   Network A          Network B           Network C
┌───────────┐      ┌───────────┐       ┌───────────┐
│   Host A  │      │  Router   │       │   Host B  │
│192.168.1.5├──────┤  1  ║  2  ├───────┤10.0.0.100 │
└───────────┘      └─────╨─────┘       └───────────┘

IP handles: addressing, routing, fragmentation
IP doesn't handle: reliability, ordering, delivery confirmation

Other Internet Layer Protocols:

• ICMP (Internet Control Message Protocol): Error reporting, ping
• ARP (Address Resolution Protocol): Finds MAC address for an IP
• IGMP (Internet Group Management Protocol): Multicast group membership

Key Characteristics of IP:

• Connectionless: Each packet is independent
• Best-effort: No guarantee of delivery
• Unreliable: Packets can be lost, duplicated, or reordered

This might seem like a weakness, but it’s actually a feature. By keeping IP simple, it can be fast and widely implemented. Reliability can be added at higher layers when needed.

Layer 3: Transport Layer

The transport layer provides end-to-end communication between applications. It’s where we choose between reliability and speed.

TCP (Transmission Control Protocol)

TCP provides reliable, ordered, error-checked delivery.

TCP Provides:
✓ Connection-oriented (explicit setup and teardown)
✓ Reliable delivery (acknowledgments, retransmission)
✓ Ordered delivery (sequence numbers)
✓ Flow control (don't overwhelm the receiver)
✓ Congestion control (don't overwhelm the network)

TCP Costs:
✗ Connection overhead (handshake latency)
✗ Head-of-line blocking (one lost packet stalls everything)
✗ Higher latency than UDP

UDP (User Datagram Protocol)

UDP provides minimal transport services—just multiplexing and checksums.

UDP Provides:
✓ Connectionless (no setup overhead)
✓ Fast (minimal processing)
✓ No head-of-line blocking
✓ Optional checksum

UDP Lacks:
✗ No reliability (packets can be lost)
✗ No ordering (packets can arrive out of order)
✗ No flow control
✗ No congestion control

When to use which?

*Modern streaming often uses TCP or QUIC for adaptive bitrate streaming.

Layer 4: Application Layer

The application layer is where user-facing protocols live. It combines the application, presentation, and session layers from OSI.

Common Application Layer Protocols:

┌─────────────────────────────────────────────────────────────┐
│                    Application Layer                         │
├──────────────┬──────────────┬──────────────┬───────────────┤
│     HTTP     │     DNS      │     SMTP     │     SSH       │
│   (Web)      │   (Names)    │   (Email)    │   (Secure     │
│              │              │              │    Shell)     │
├──────────────┼──────────────┼──────────────┼───────────────┤
│     FTP      │    DHCP      │    SNMP      │     NTP       │
│   (Files)    │   (Config)   │ (Management) │    (Time)     │
└──────────────┴──────────────┴──────────────┴───────────────┘

This layer handles:

• Data formatting and encoding
• Session management
• Application-specific protocols
• User authentication (in many protocols)

Putting It All Together

Let’s trace what happens when you request a webpage:

1. APPLICATION LAYER
   Your browser creates an HTTP request:
   "GET /index.html HTTP/1.1"

2. TRANSPORT LAYER
   TCP segments the data, adds:
   - Source port (e.g., 52431)
   - Destination port (80)
   - Sequence number
   - Checksum

3. INTERNET LAYER
   IP adds:
   - Source IP (192.168.1.100)
   - Destination IP (93.184.216.34)
   - TTL (Time to Live)

4. NETWORK ACCESS LAYER
   Ethernet adds:
   - Source MAC
   - Destination MAC (router's MAC)
   - Frame check sequence

5. PHYSICAL
   Converted to electrical signals on the wire

On the receiving end, this process reverses—each layer strips its header and passes data up.

The Protocol Graph

Rather than a strict stack, TCP/IP is better visualized as a graph:

                 ┌─────────────────────────────────────┐
                 │           Applications              │
                 │  HTTP   SMTP   DNS   SSH   Custom   │
                 └──────────────┬──────────────────────┘
                                │
              ┌─────────────────┴─────────────────┐
              │                                   │
         ┌────┴────┐                        ┌────┴────┐
         │   TCP   │                        │   UDP   │
         └────┬────┘                        └────┬────┘
              │                                   │
              └─────────────────┬─────────────────┘
                                │
                          ┌─────┴─────┐
                          │    IP     │
                          └─────┬─────┘
                                │
         ┌────────────────┬─────┴─────┬────────────────┐
         │                │           │                │
    ┌────┴────┐     ┌────┴────┐ ┌────┴────┐     ┌────┴────┐
    │Ethernet │     │  WiFi   │ │Cellular │     │  Other  │
    └─────────┘     └─────────┘ └─────────┘     └─────────┘

Any application can use TCP or UDP. Both use IP. IP can run over any network technology. This flexibility is why the internet works.

Why TCP/IP Won

The OSI model was designed by committee to be complete and correct. TCP/IP was designed by engineers to work. Key differences:

TCP/IP’s success came from:

1. Working code first: The spec described implementations that already worked
2. Simplicity: Fewer layers, clearer responsibilities
3. Flexibility: “Be liberal in what you accept, conservative in what you send”
4. Open standards: Anyone could implement it

Summary

The TCP/IP model is the practical foundation of the internet:

Understanding this model—especially the separation between IP (best-effort routing) and TCP (reliable delivery)—is essential for understanding how the internet works.

Next, we’ll look at how data is wrapped and unwrapped as it moves through these layers: encapsulation.