PDU Names (Segment / Packet / Frame / Bit)

A PDU (Protocol Data Unit) is the name given to data at each layer. As data travels down the layers, each layer wraps it and the "chunk" gets a new name.

OSI Layer	TCP/IP Layer	PDU Name
Application (L7), Presentation (L6), Session (L5)	Application	Message / Data
Transport (L4)	Transport	Segment (TCP) / Datagram (UDP)
Network (L3)	Internet	Packet
Data Link (L2), Physical (L1)	Network Access	Frame / Bits

What's Inside Each PDU? (Russian Nesting Dolls)

BITS
 └── FRAME   [L2 Header + L2 Trailer]
      └── PACKET  [L3 Header]
           └── SEGMENT [L4 Header]
                └── DATA (actual payload)

Segment: [ Source Port | Dest Port | Seq No. | Ack No. | Flags | Data ]
Packet:  [ Source IP | Dest IP | TTL | Protocol | Segment ]
Frame:   [ Dest MAC | Src MAC | Type | Packet | FCS ]

Breaking Down Each PDU

Message / Data (L5–L7)

• The raw application data.
• Example: an HTTP request like GET /index.html.
• No network headers yet — just pure content.

Segment (L4 — TCP)

• Adds port numbers (who's talking to whom on a device).
• Adds sequence numbers (for ordering and reassembly).
• TCP specific — if UDP, it's called a Datagram (much smaller header).

Packet (L3)

• Adds IP addresses (where it's coming from, where it's going).
• TTL (Time To Live) — decrements at each router, dies at 0 (prevents infinite loops).
• Protocol field says what's inside (TCP=6, UDP=17, ICMP=1).

Frame (L2)

• Adds MAC addresses (local delivery within a network).
• FCS (Frame Check Sequence) — trailer for error detection.
• Only valid for the current network hop — gets replaced at every router.

Bits (L1)

• Frame converted to electrical signals / light pulses / radio waves.
• Just 0s and 1s flying across the wire.

★ Important Interview Point — Frame is Hop-to-Hop, Packet is End-to-End

Path: Laptop → Router1 → Router2 → Server

	Packet (L3)	Frame (L2)
Source IP	Your laptop's IP	Changes at each hop
Dest IP	Server's IP	Changes at each hop
Source MAC	—	Your laptop → R1 → R2
Dest MAC	—	R1 → R2 → Server

• Packet stays the SAME end-to-end (IPs don't change).
• Frame is RECREATED at every hop (MACs change each time).

This is why routers need both L2 and L3 — they strip the old frame, read the packet's destination IP, then create a brand new frame for the next hop.

Mnemonic: Do Some People Fear Birthdays? → Data → Segment → Packet → Frame → Bits