Three widely used transport layers:
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TCP (reliable bi-directional byte stream), UDP(unreliable delivery of datagrams between applications), ICMP（detect when things go wrong, e.g. ping, traceroute）
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End-to-end principle: 功能放到断点，giving mild reason and strong reason
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How TCP builds up reliable connections:
- data errors and missing packets
- sliding window
- retransmission
- TCP finite state machine, tracks current status and transmission