11 — Practice solutions
These are independently written explanations, not an official marking scheme.
11A
TCP establishes a connection using SYN, SYN+ACK, ACK. It provides ordered reliable delivery using sequence numbers, acknowledgements and retransmission mechanisms. UDP sends datagrams without that handshake and does not itself guarantee delivery/order. ACK 801 indicates that the receiver next expects byte 801 after contiguous receipt of preceding stream data; it is not “packet 801 was received”.
Reasoning
Give a comparison on each requested dimension—establishment and reliability—then the three messages and the ACK interpretation. “Connection-oriented” alone does not explain error recovery. An acknowledgement is measured against the byte sequence, not a packet count.
11B
DHCP provides the laptop’s network configuration: client Discover, server Offer, client Request, server Acknowledge confirming the lease. DNS resolves the website’s domain name to an appropriate address record so traffic can be directed towards the server. DNS does not perform DHCP’s address leasing for the laptop.
Reasoning
The laptop needs its own configuration before it can communicate normally; later it needs the address associated with the requested website. DHCP supplies the former and DNS resolves the latter. State the sender and purpose of each DHCP message, not just the letters DORA.
11C
Sender: 1111 + 0001 + 0010 = 1 0010
Wrap: 0010 + 0001 = 0011
Invert: 1100 ← checksum
Receiver: 15 + 1 + 2 + 12 = 1 1110
Wrap: 1110 + 0001 = 1111
Invert: 0000 ← check passesSwapping the first two words preserves their sum, so the check still passes. This checksum does not establish correct word order or absence of every possible error.
11D
The address is leased temporarily, so the device seeks renewal to keep using it. If the lease expires without renewal, it must stop using that address and obtain valid configuration again; otherwise the address might be reallocated and conflict. Renewal is not the same action as explicitly releasing a lease.