Data packets

Question 1

The packets of data are usually quite small, typically

Answer 1

64KiB,

Question 2

Packet structure
A typical packet is split up into:

Answer 2

» a packet header
» the payload
» a trailer.

Question 3

Header

Answer 3

IP address of
the source
device (sender)

IP address of
the destination
device (receiver)

Size of the
packet (in
bytes)

Sequence
number of
the packet
(allowing correct
reassembly)

Question 4

Payload

Answer 4

The actual data
in the packets
(~64 KiB) – the
payload

Question 5

Trailer

Answer 5

Method of
identifying the
end of the
packet

Some form of
error checking
to ensure
packet arrives
error-free

Question 6

For each packet, the packet header consists of:

Answer 6

» the IP address of the sending device
» the IP address of the receiving device
» the sequence number of the packet (this is to ensure that all the packets can
be reassembled into the correct order once they reach the destination)
» packet size (this is to ensure the receiving station can check if all of the
packets have arrived intact).

Question 7

Why is it important to identify the end of a packet in a packet trailer?

Answer 7

It ensures that packets can be separated from each other as they travel from the sending to the receiving station.

Question 8

What is the role of cyclic redundancy checks (CRCs) in packet transmission?

Answer 8

CRCs check for transmission errors by comparing the number of 1-bits in the payload at both the sending and receiving computers.

Question 9

How does a sending computer use CRCs?

Answer 9

The sending computer adds up all the 1-bits in the payload and stores the result as a hex value in the packet trailer before sending the packet.

Question 10

What does the receiving computer do with CRCs when a packet arrives?

Answer 10

The receiving computer recalculates the number of 1-bits in the payload and checks this value against the one stored in the packet trailer.