← IGCSE Computer Science Paper 1

Labelling the structure of a packet

A packet has a header (destination address, originator's address, packet number), a payload (the data) and a trailer (marks the end). Label each item by the part it belongs to.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 14

Understanding Labelling the structure of a packet in IGCSE Computer Science Paper 1

What it does: The trailer is the last part of a packet. It contains an end-of-packet marker so the receiver knows the packet is complete. Packet number is one of the three header fields. The receiving device uses it to reorder packets that arrive out of sequence. The payload is the data itself. The header only holds information about the packet. The trailer comes after the payload and shows that the packet has finished.

Common mistakes: The originator's address is one of the three header fields, not part of the payload. It should be header.originator = 10.0.0.2. The packet number is a header field, used to put packets back in order. It should be labelled HEADER: packet no. 3.

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 110–120) and word search (Modules 86–120).

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Learning

Duo

A packet carries the destination IP address. Which part holds it?

TrailerHeader

Matches

Payload: dest 192.168.1.5, packet 3

↓

The destination address and packet number are header fields, not payload. The payload holds only the data being sent.

Applying

Meanings

Term

header, payload, trailer

Meaning

A packet has a header (destination address, originator's address, packet number), a payload (the data) and a trailer (marks the end). Label each item by the part it belongs to.

Practice

HEADER: destination address 192.168.0.5

Testing

Gaps

: dest=10.0.0.5, from=10.0.0.2, num=3

Check Answer

Spots

1header.destination = 10.0.0.7

2payload.originator = 10.0.0.2

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Quick Questions

1. A packet carries the destination IP address. Which part holds it?

A. Header ✓

B. Trailer

The destination address is one of the three header fields, so the router knows where to send the packet.

2. Which part of a packet holds the actual data being sent, such as a piece of an email?

A. Header

B. Payload ✓

The payload is the data itself. The header only holds information about the packet.

3. A packet is labelled 'Packet 3 of 5'. Which part holds the packet number?

A. Header ✓

B. Payload

The packet number is a header field. The destination can use it to put the packets back in order.

4. Which part of a packet marks where the packet ends?

A. Header

B. Trailer ✓

The trailer comes after the payload and shows that the packet has finished.

5. How many fields does a packet header contain in this course?

A. Three: destination address, originator's address and packet number ✓

B. Two: destination address and payload

The header has three fields. The payload is a separate part of the packet and is not inside the header.

Match the Pairs

Header: dest 192.168.1.5, from 10.0.0.2, packet 3 → The header holds the destination address, the originator's address and the packet number

Payload: "Hello, are you free?" → The payload is the actual data being sent, such as part of a message

The trailer comes at the end of the packet and marks where the packet finishes → Trailer: END

The packet number is a header field that lets the destination put packets back in order → Header: packet number = 3 of 8

Payload: dest 192.168.1.5, packet 3 → The destination address and packet number are header fields, not payload. The payload holds only the data being sent.

Trailer: from 10.0.0.2, dest 192.168.1.5 → The originator's and destination addresses are header fields. The trailer only marks the end of the packet.

Key Term

header, payload, trailer

A packet has a header (destination address, originator's address, packet number), a payload (the data) and a trailer (marks the end). Label each item by the part it belongs to.

Fill in the Blanks

1. HEADER: dest=10.0.0.5, from=10.0.0.2, num=3 PAYLOAD: "Hi Sam" TRAILER: END

Hint: Type the label of the part that holds the addresses and the packet number.

2. HEADER: dest=10.0.0.5, from=10.0.0.2, num=3 PAYLOAD: "Hi Sam" TRAILER: END

Hint: Type the label of the part that carries the actual data being sent.

3. HEADER: dest=10.0.0.5, from=10.0.0.2, num=3 PAYLOAD: "Hi Sam" TRAILER: END

Hint: Type the label of the part that marks the end of the packet.

4. HEADER: dest=10.0.0.9, from=10.0.0.4, num=2 PAYLOAD: "Meet at 5" TRAILER: END

Hint: Type the header field name that stores the originator's address.

5. HEADER: dest=10.0.0.9, from=10.0.0.4, num=2 PAYLOAD: "Meet at 5" TRAILER: END

Hint: Type the header field name that stores the packet number.

Spot the Error

Question 1

A header.destination = 10.0.0.7

B payload.originator = 10.0.0.2

C header.packet_no = 2

D payload = 'Hi Sam'

E trailer = 'END'

Line B — The originator's address is one of the three header fields, not part of the payload. It should be header.originator = 10.0.0.2.

Question 2

A HEADER: dest 172.16.0.4

B HEADER: from 172.16.0.9

C PAYLOAD: packet no. 3

D PAYLOAD: Meet at 5

E TRAILER: end marker

Line C — The packet number is a header field, used to put packets back in order. It should be labelled HEADER: packet no. 3.

Question 3

A HEADER: dest 192.168.1.20

B HEADER: from 192.168.1.8

C HEADER: packet no. 1

D TRAILER: Hello Mia

E TRAILER: end of packet

Line D — The message data is the payload. The trailer only marks the end of the packet. The line should be PAYLOAD: Hello Mia.

Question 4

A packet = []

B packet.add(trailer)

C packet.add(payload)

D packet.add(trailer)

Line B — The order of a packet is header, then payload, then trailer. The trailer must come last, so line 2 should add the header.

Question 5

A payload = 'Lunch at 1'

B trailer = 'END'

C header = [dest, origin, END]

Line C — The third header field is the packet number, not the end marker. The END marker belongs in the trailer. It should be [dest, origin, num].

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