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CAIE 9618 · Paper 1 · Topic 1.2.5

TCP/IP
Model

Four Layers · Protocols at Each Layer · Encapsulation · Decapsulation

CSZone Cambridge International AS & A Level Computer Science 9618
Learning Objectives

By the end of this video you will be able to...

Name and describe the four layers of the TCP/IP model
State protocols used at each layer and explain their purpose
Explain encapsulation and decapsulation
Explain why a layered model is beneficial for network design
The Four Layers

TCP/IP Model Overview

LAYER 4: APPLICATION
User-facing services · HTTP, HTTPS, FTP, SMTP, POP3, IMAP, DNS
LAYER 3: TRANSPORT
End-to-end delivery · TCP (reliable) · UDP (fast, unreliable) · Port numbers
LAYER 2: INTERNET
Routing between networks · IP (IPv4, IPv6) · Packets · Routers
LAYER 1: LINK
Physical transmission · Ethernet, Wi-Fi · Frames · Switches, NICs
TCP vs UDP

Transport Layer Protocols

TCP — Transmission Control Protocol
Connection-oriented (3-way handshake)
Guaranteed delivery — acknowledgements
Retransmits lost packets
Slower but reliable
Used by: HTTP, HTTPS, FTP, SMTP
UDP — User Datagram Protocol
Connectionless — no handshake
No guaranteed delivery
No retransmission
Faster but unreliable
Used by: DNS, streaming, VoIP, gaming
Encapsulation

Adding Headers at Each Layer

Encapsulation: as data travels down the layers, each layer adds its own header (and sometimes a trailer) to the data — wrapping it like an envelope within an envelope.
Application
Data (the actual message content)
Transport
[TCP/UDP header] + Data → Segment
Internet
[IP header] + Segment → Packet
Link
[Frame header] + Packet + [trailer] → Frame
Why Use a Layered Model?

Benefits of the TCP/IP Layered Approach

Modularity — each layer can be developed/updated independently
Interoperability — different manufacturers can implement the same layer
Fault isolation — easier to identify which layer has a problem
Simplification — each layer only needs to know about adjacent layers
💡 If Wi-Fi is replaced by a new technology, only the Link layer changes — all other layers remain the same
Exam Practice

Cambridge-style questions

Question 1
Name and describe the function of each of the four layers of the TCP/IP model.
4 marks
✓ MARK SCHEME
1 mark
Application: provides user-facing services; protocols such as HTTP, FTP, SMTP
1 mark
Transport: end-to-end communication; TCP or UDP; port numbers
1 mark
Internet: routing packets across networks; IP addresses; routers
1 mark
Link: physical transmission of frames on a network; Ethernet, Wi-Fi; MAC addresses
Exam Practice
Question 2
Explain the difference between TCP and UDP. For each, give a suitable application where that protocol is preferred.
4 marks
1 mark
TCP: reliable, connection-oriented — guarantees delivery and order of data via acknowledgements
1 mark
Application: web browsing (HTTP/HTTPS), email (SMTP), file transfer (FTP)
1 mark
UDP: unreliable, connectionless — sends data without guaranteeing delivery, faster
1 mark
Application: video streaming, online gaming, VoIP, DNS lookups
Common Mistakes

Don't lose easy marks

1
Mixing up the layer order — the TCP/IP model goes Application (top) → Transport → Internet → Link (bottom). Data travels DOWN from application to link when sending, UP from link to application when receiving.
2
Saying UDP is used for web browsing because "it's faster" — web browsers use TCP not UDP because web pages must be delivered completely and in order. Speed alone doesn't determine which to use — reliability does.
3
Confusing the TCP/IP model with the OSI model (7 layers) — Cambridge 9618 uses the TCP/IP model (4 layers), NOT the OSI model. Always refer to Application, Transport, Internet, Link.
Topic Summary — 1.2.5

What You Need to Know

FOUR LAYERS
Application: HTTP, FTP, SMTP, DNS
Transport: TCP (reliable), UDP (fast)
Internet: IP, routing, packets
Link: Ethernet, Wi-Fi, frames, MAC
TCP vs UDP
TCP: reliable, ordered, ACKs, slower
UDP: unreliable, no ACKs, faster
TCP for: HTTP, FTP, email
UDP for: streaming, gaming, VoIP
ENCAPSULATION
Each layer adds its own header
App data → Segment → Packet → Frame
Decapsulation: headers removed going up
Benefits: modularity, interoperability
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Next Video

1.2.6
Circuit & Packet Switching and DNS
How Data is Routed · How Names Become Addresses
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