Explain how packet switching is used to transfer messages across the internet.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

1. [5] One mark for each correct marking point (Max 5)
2. • A large message is divided up into a group of smaller chunks of the same size called packets
3. • The packet has a header and a payload
4. • The header contains a source IP address, destination IP address (and sequence number)
5. • Each packet is dispatched independently
6. • ... and may travel along different routes / paths
7. • The packets may arrive out of order
8. • ... and are reassembled into the original message at the destination
9. • If packets are missing / corrupted a re-transmission request is sent.
10. [3] One mark for each correct marking point (Max 3)
11. • The router examines the packet's header
12. • It reads the IP address of the destination (from the packet header)
13. • A router has access to a routing table
14. • ...containing information about, e.g., available hops / netmask / gateway used
15. • ... and the status of the routes along the route
16. • ... the router decides on the next hop / best route
17. • ... and sends the packet on its next hop.