Ports, TCP and UDP

An IP address gets a packet to a machine, and the transport layer gets it from there to the right program. The choice between its two protocols, TCP and UDP, decides whether an application waits for lost data or carries on without it, which shapes how fast it feels and what it can promise.

The topic starts with ports and the five values that name a connection, then describes UDP and TCP in turn, compares them, and applies the choice to three different applications. A gotcha separates the two errors a failed connection most often gives.

How TCP achieves its reliability, the handshake, acknowledgements and congestion control, is the next topic.

By the end you can identify a connection by its five values, name the common ports, choose a transport for an application, and tell "connection refused" from "connection timed out".