11. TCP, UDP, checksums and application protocols - revision mindmap
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TCP versus UDP
- Both transport protocols with ports
- TCP: connection state, reliable ordered byte stream
- UDP: datagrams without its own delivery/order guarantee
- Choose by loss, delay and ordering requirements
TCP lifecycle
- Establish: SYN → SYN+ACK → ACK
- Separate sequence and acknowledgement fields
- ACK: next byte expected; retransmission and ordering
- Flow control and taught FIN/ACK termination patterns
Checksum and parity
- Corruption versus loss versus reordering
- Add fixed-width words with end-around carry
- Complement sender sum; receiver includes checksum
- Pass does not guarantee correctness; parity detects odd flips
Address configuration
- DHCP: IP, mask, gateway and DNS information; server port 67, client port 68.
- Discover → Offer → Request → Acknowledge
- Lease, renewal, expiry and release distinctions
- Static reservation versus dynamic pool
Application services
- DNS (53): resolver, cache, hierarchy, authoritative answer.
- HTTP (80) / HTTPS (443): request/response; HTTPS uses TLS.
- SMTP (25) sends; POP3 (110) / IMAP (143) retrieve or manage mail.
- FTP transfers files; control port 21. Secure variants can use other ports.
Worked example & exam traps
- 4-bit checksum: 1101 + 1011 = 11000 → 1000 + 1 = 1001.
- Complement 1001 → 0110 checksum; 1001 + 0110 = 1111.
- Receiver complement = 0000: no error detected, not proof of no error.
- Avoid: DNS leases IPs; SYN-ACK is only ACK; checksum fixes loss
Sources and reading
This map condenses Chapter 11 and its source trail. Use the self-learning reading guide for the corresponding VJC pages and A notes. It does not add topics to the stated promo scope.