TCP vs UDP, and the 3-way handshake

The same thing, byte by byte (optional): TCP Handshake - the same picture zoomed into real bytes, off one actual connection this laptop made to a real website.

Before you start. You need to know that every device on a network has an address, and that a message crossing the internet travels broken into chunks rather than as one stream. Nothing else is assumed - what those chunks look like, and what promises come with them, is this whole lesson.

The idea in 30 seconds

A packet is a chunk of data with a return address anyone could fake - the network never checks who really sent it.
A port is a number saying which program on a machine a packet is for; the address gets a packet to the machine, the port gets it to the program.
UDP is a postcard: no setup, no check that anyone’s listening, no guarantee anything arrives or arrives in order.
TCP is a phone call: a three-message handshake happens before any real data moves, faking a reliable line on top of a network that promises no such thing.
The handshake works because each side must echo back a starting number only a real listener could have heard - the same number a spoofer, unable to hear the reply, has to guess.
That handshake is not free: one message there and one back (a round trip) before the first useful byte, every time.

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