The internet’s basic delivery service promises nothing. Packets can be lost, duplicated, delayed, or turn up in the wrong order, and none of that counts as a fault. Yet the file you download arrives whole and the message you send arrives intact, because TCP sits on top of that best-effort delivery and does the tidying up.
TCP numbers the bytes it sends, waits for acknowledgements, resends anything that goes missing, and reassembles what arrives into the right order before handing it to the application. It also paces itself, slowing down when the network shows signs of congestion. The clean, ordered stream you think you have is a reconstruction at the receiving end.
Three things to remember
- IP delivery is best-effort: loss, duplication and reordering are normal behaviour, not errors.
- Sequence numbers and acknowledgements are how TCP works out what went missing and resends it.
- The ordered stream is assembled by the receiver. The network never carried one.
Related explanations
- What Happens When You Play a Game Online? (It Throws Your Data Away)
- What Happens When You Hit Enter? (It’s Always DNS)
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