TCP/IP
The layered protocol suite underneath essentially all application traffic — IP routes packets, TCP turns them into a reliable ordered stream.
IP moves packets between addresses and guarantees nothing: packets may be dropped, duplicated, reordered or delayed. TCP builds a reliable ordered byte stream on top, using sequence numbers, acknowledgements, retransmission and flow control.
Three consequences that show up in architecture rather than in networking class:
Connection setup costs a round trip (plus one or two more for TLS), which is why connection pooling and HTTP keep-alive matter so much on high-latency links, and why a service that opens a new connection per request is slow for reasons no profiler will show you.
Congestion control means throughput ramps up, so a short-lived connection never reaches full speed — another argument for pooling.
Head-of-line blocking: one lost packet stalls everything behind it on that connection. HTTP/2 multiplexes streams over one TCP connection and therefore still suffers this; HTTP/3 moves to QUIC over UDP specifically to escape it.