📘 04-01: The TCP/IP Suite & Encapsulation¶

❓ What Is TCP/IP?¶

TCP/IP is a suite of cooperating protocols — not a single protocol. Core members:

Protocol	Layer	Role
HTTP, DNS, SMTP	Application	Services
TCP, UDP	Transport	Delivery + ports
IP (v4/v6), ICMP	Network	Addressing + routing
ARP, Ethernet	Link	Local delivery + framing

📌 The TCP/IP Model vs. OSI¶

TCP/IP (4-layer)	OSI layers
Application	5, 6, 7
Transport	4
Internet	3
Link (Network Access)	1, 2

📌 Encapsulation (Data Moving Down the Stack)¶

Each layer adds a header (Data Link also adds a trailer):

[App data]                                   ← payload (L7-5)
[TCP/UDP hdr | data]              = Segment   ← ports (L4)
[IP hdr | segment]               = Packet    ← IP addresses (L3)
[Frame hdr | packet | trailer]   = Frame     ← MAC addresses (L2)
[bits on the wire]               = Bits       (L1)

The receiving host reverses this (decapsulation), stripping one header per layer going up.

📌 What Each Header Adds¶

Layer	Header adds
Transport	Source/destination ports
Network	Source/destination IP + TTL + protocol #
Data Link	Source/destination MAC + FCS (in trailer)

Tip

Key idea — TCP/IP is a suite. As data goes down the stack it's encapsulated — ports at L4, IPs at L3, MACs at L2 — becoming segment → packet → frame → bits.