I talked about TCP and UDP in this post. Now we’re going to talk about how TCP really works, how it keeps that connection going. It’s how mail works, it’s how a lot of web traffic works, so how does that pipe keep going?

A pipe

Well, it isn’t a physical pipe like that picture. It isn’t a physical connection at all and it also isn’t a psychic connection, instead, it’s a virtual connection.

It starts by the two systems, let’s call them Captain America and Black Panther, negotiating a connection.  The connection is called a handshake.

A handshake

Actually it’s called a three-way handshake and it works like this:

Captain America tells Black Panther, “Hey, I want to talk to you.”. He does this by sending a packet called a SYN packet.

Black Panther then tells Captain America, “Gotcha! Good idea, Let’s talk.” He does this by sending back a packet called an ACK. The three-way is because Captain America acknowledges Black Panther’s agreement to talk by sending back his own ACK.

The TCP connection uses these three packets to set the connection up, SYN, ACK and ACK.

So they’re talking away, but like in all conversations, sometimes there’s a lull where neither side says anything. Maybe they’re each getting a nice drink, maybe they’re each writing a note to themselves to make fun of Iron Man, maybe they’re just taking a nap. The connection sends back and forth KEEP ALIVE packets to say ‘Hey, you still there?’ and the other side acknowledges ‘Yup, still here. Taking a nap, be back to talking with you in a minute.’

All conversations eventually end, or at least take enough of a break that you hang up the phone and do something else for a while. When this happens, the TCP connection is shutdown, like when you hang up your phone. Your phone tells the other guy ‘yup, I’m done’ and shuts down the communication. Similarly, the one end of the TCP connection will tell the other ‘I’m outta here’, which ends the connection.