📡 Paper 1 · 1.2 Communication and Internet Technologies
1.2.5 TCP/IP Model and Layers
Cambridge 9618 · International A Level Computer Science · ~15 min read
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Why Use a Layered Model?

Networking is complex. A layered model breaks the task of network communication into distinct layers, each with a specific responsibility. Benefits include:

  • Standards allow different manufacturers' equipment to work together (interoperability)
  • Each layer can be developed and updated independently
  • Easier to troubleshoot — isolate which layer has the problem
  • Separation of concerns — each layer uses the services of the layer below

The TCP/IP Model (4 Layers)

Cambridge 9618 focuses on the TCP/IP model (also called the Internet model), which has four layers:

4. Application Layer
Protocols: HTTP, HTTPS, FTP, SMTP, POP3, IMAP, DNS, SSH

Provides network services directly to the application. Handles high-level protocols and data representation.

3. Transport Layer
Protocols: TCP (reliable), UDP (fast/unreliable)

Provides end-to-end communication. TCP: connection-oriented, ensures delivery, flow/error control. UDP: connectionless, no guarantee of delivery, faster.

2. Internet / Network Layer
Protocols: IP (IPv4/IPv6), ICMP

Handles logical addressing (IP addresses) and routing of packets across networks. Each packet may take a different route.

1. Network Access / Link Layer
Protocols: Ethernet, Wi-Fi (IEEE 802.11), ARP

Handles physical transmission — hardware addressing (MAC), framing, error detection at the physical level. Deals with the actual network hardware.

TCP vs UDP

FeatureTCP (Transmission Control Protocol)UDP (User Datagram Protocol)
Connection typeConnection-oriented (establishes connection before data)Connectionless (sends without establishing connection)
ReliabilityGuaranteed delivery — ACKs, retransmissionNo guarantee — packets may be lost
OrderPackets reassembled in correct orderNo ordering — may arrive out of sequence
SpeedSlower (overhead of reliability)Faster (no acknowledgement overhead)
Error checkingYes — flow control and error correctionMinimal — checksum only
Use casesWeb browsing, email, file transfer (correctness crucial)Video streaming, VoIP, online gaming, DNS

The Three-Way Handshake (TCP)

TCP establishes a connection using a three-way handshake before data is transmitted:

  • SYN: Client sends a synchronise packet to the server requesting connection
  • SYN-ACK: Server responds with synchronise-acknowledge, confirming and sending its own SYN
  • ACK: Client acknowledges — connection established; data transfer begins

Encapsulation and Decapsulation

As data passes down through the TCP/IP layers (sending), each layer adds its own header (and sometimes a footer/trailer). This process is called encapsulation.

  • Application: data → message
  • Transport: adds TCP/UDP header → segment (TCP) or datagram (UDP)
  • Internet: adds IP header with source/destination IP → packet
  • Link: adds MAC header + trailer → frame

At the receiving end, each layer removes its header — called decapsulation — until the original data is restored.

Exam tip: Cambridge frequently asks to match protocols to layers, compare TCP and UDP, or explain the purpose of each layer. Memorise the 4 layers and which protocols belong to each. Key fact: TCP = reliable, connection-oriented; UDP = fast, connectionless. Also know the three-way handshake and encapsulation.
⚠️ Common Mistakes
  • Confusing the TCP/IP model with the OSI model (7 layers) — Cambridge 9618 uses the 4-layer TCP/IP model
  • Saying UDP is unreliable and therefore useless — it is used intentionally where speed matters more than guaranteed delivery (streaming, gaming)
  • Thinking IP handles reliability — IP is best-effort; TCP at the transport layer handles reliability
  • Confusing MAC addresses (link layer) with IP addresses (internet layer)
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Worksheet — 1.2.5 TCP/IP Model

8 questions · instantly marked · Cambridge 9618 standard

Q1Name the four layers of the TCP/IP model, from highest to lowest.[4]
✅ Mark scheme
Mark scheme
1 mark each: Application [1]; Transport [1]; Internet (Network) [1]; Network Access / Link [1].
Q2State two advantages of using a layered model for network communication.[4]
✅ Mark scheme
Mark scheme
2 marks each for any two: Different manufacturers can develop equipment that works together because each layer uses agreed standards (interoperability) [2]; layers can be developed or updated independently without affecting other layers [2]; easier to troubleshoot — can isolate which layer a problem occurs at [2].
Q3Compare TCP and UDP. State one scenario where each is more appropriate.[4]
✅ Mark scheme
Mark scheme
TCP: connection-oriented, guaranteed delivery with acknowledgements and retransmission, slower — appropriate for web browsing/email/file transfer where correctness is critical [2]; UDP: connectionless, no guaranteed delivery, faster — appropriate for live video streaming/VoIP/online gaming where speed matters more than perfect delivery [2].
Q4Describe the TCP three-way handshake.[3]
✅ Mark scheme
Mark scheme
Client sends a SYN (synchronise) packet to the server to request connection [1]; server responds with SYN-ACK (synchronise-acknowledge) confirming it received the request and sending its own SYN [1]; client responds with ACK (acknowledge) — connection established and data transfer begins [1].
Q5Name the protocol at the Internet layer. State its main function.[2]
✅ Mark scheme
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IP (Internet Protocol) [1]; it handles logical addressing (IP addresses) and the routing of data packets across networks — determining the best path for each packet [1].
Q6Explain the process of encapsulation as data moves down the TCP/IP layers.[4]
✅ Mark scheme
Mark scheme
At each layer, a header (and sometimes trailer) is added to the data from the layer above [1]; Transport layer adds TCP/UDP header — creates a segment/datagram [1]; Internet layer adds IP header with source and destination IP addresses — creates a packet [1]; Link layer adds MAC header and trailer — creates a frame for physical transmission [1].
Q7At which layer of the TCP/IP model does a router operate? What address does it use?[2]
✅ Mark scheme
Mark scheme
A router operates at the Internet (Network) layer [1]; it uses IP addresses to route packets between networks [1].
Q8Explain why the Network Access layer handles MAC addresses while the Internet layer handles IP addresses.[3]
✅ Mark scheme
Mark scheme
MAC addresses are hardware/physical addresses used only within a local area network segment [1]; switches on the LAN use MAC addresses to deliver frames between devices on the same network [1]; IP addresses are logical addresses used for routing between different networks — a router strips the frame (removing MAC addressing) and re-encapsulates using the next hop's MAC address [1].
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Mini Test — 1.2.5 TCP/IP Model

10 questions · 10 marks · 10 minutes

← 1.2.4 Internet Protocols
12 of 82 · Cambridge 9618
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