The big idea: A protocol is an agreed set of rules for a conversation: what gets said, in what order, and what a reply means.
Two machines built by different companies can talk only because both follow the same rules.
Two jobs, two groups: Transport protocols decide how the data gets there — whole and in order, or fast.
Application protocols decide what the conversation is actually about — a page, a secure page, an address.
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The one difference that matters: TCP checks that everything arrived and asks again for anything missing.
UDP does not check at all. It sends and moves on.
TCP
- Numbers the packets and checks they all arrived
- Asks again for any that went missing
- Puts them back in order at the far end
- Slower, and the checking adds traffic
- Web pages, email, file transfers — anything that must be complete
UDP
- Sends without checking, and never resends
- No delay waiting for confirmations
- Packets may arrive late, out of order, or not at all
- Much less overhead
- Live video, voice calls, online games — anything where late is worse than missing
Why a video call uses the sloppy one: In a call, a packet that arrives late is useless — that moment of sound has already passed.
Resending it would only delay everything after it. Losing it costs a tiny glitch; waiting for it costs the whole conversation.
Step to Transport to see where TCP and UDP sit, and to Application for the three below.
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HTTP — fetching a page
HTTPS — the same thing, encrypted
DHCP — being given an address
HTTPS is two guarantees, not one: Encryption — nobody in between can read the conversation.
Identity — the certificate proves the server is who it claims to be. A padlock without the second guarantee would be worthless.
How this is tested — you must choose the protocol from what the application needs, and justify it. It comes up two ways:
Paper 1 Section A
- Describe a named protocol, 1-3 marks
- Choose TCP or UDP for a stated application
- Explain what HTTPS adds to HTTP
Paper 1 Section B — case study
- Identify the protocols the case study relies on
- Justify them from what that system does
The classic trap: Saying "TCP is better than UDP". TCP is better when data must be complete; UDP is better when it must be timely. A live call deliberately uses the one with no guarantees.
A company's website lets customers download invoices and also offers live video support. Explain which transport protocol suits each, and why.
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