TCP and UDP

IP moves packets between hosts. Applications need one more layer to deliver data to the right program. TCP and UDP provide that transport layer.

Both protocols use port numbers. A port number identifies an application endpoint within a host. The combination of IP addresses, transport protocol, and port numbers lets a host direct incoming data to the correct socket.

TCP and UDP make different promises. That difference shapes application design, troubleshooting, and performance.

The central choice

TCP provides a reliable, ordered byte stream. It establishes a connection before application data flows. Sequence numbers identify bytes in the stream. Acknowledgments report received data. Retransmission repairs detected loss. Flow control protects a receiver, while congestion control adapts sending to network conditions.