Data is split into packets before it is sent across the internet.
Explain why each packet has a packet number in its header.
Mark scheme
Award up to two marks for a linked explanation such as:
- The receiver can put the packets back into the correct order (1) because packets may arrive out of order/take different routes (1)
- The receiver can see if a packet is missing (1) because the sequence of numbers shows a gap (1)
- The receiver knows when the whole file has arrived (1) because the total number of packets/last packet number is known (1)
For both marks, the expansion must follow/associate with the statement.
Allow 'so the packets can be put back in the correct order/reassembled' for the first mark, but the second mark needs the reason, i.e. packets may arrive out of order/take different routes.
Do not accept 'so the packets can be identified' without stating what the number is used for, e.g. reordering or spotting a missing packet.
Do not accept 'so the router knows where to send it' - the router uses the destination IP address, not the packet number.
Example answers and how they were marked
The receiver can put the packets back in the correct order because they may arrive out of order.
Why: Linked point (1) and reason (1): the packet number allows reordering because packets can arrive out of order.
It shows the receiver the order of the packets.
Why: The point is given (1) but there is no linked reason, such as packets arriving out of order.
So the router knows which route to use.
Why: Routers use the destination IP address, not the packet number, so this earns nothing; it is named in the mark scheme as a 'Do not accept'.