In the previous post we talked about the /etc/services file and how that told the computer what ports to use for what traffic.  In that file we saw lots of mentions of tcp and udp but there wasn’t anything that told you what those two abbreviations meant.

In the Internet, there’s different destinations for traffic and there’s also different kinds of traffic.  They’re designed for two different things.

When you send a letter by snail mail, you open up the mailbox, drop your letter in, and hope it gets there.  It usually makes it to its destination, but you have no way of making sure it does.

Mailbox

On the other hand, when you make a phone call, you know it connects (because you’re talking to the other guy) and it works until either you hang up or the connection cuts off for some reason.  You don’t always know why it got interrupted, just that something cut it off.

Telephone

We can communicate two ways, each of them designed for some specific reason.

In the Internet, udp traffic is like the mailbox.  Its traffic that just flows across the Internet with no verification that it made it.  It’s used in case when it’s not really a huge deal if the connection doesn’t work.  Network Time Protocol is an example of udp traffic.  It isn’t great if the time update doesn’t make it, but it isn’t the end of the world either.

On the other hand, tcp traffic is like the telephone call.  In tcp traffic, a connection is created between two systems and it is kept alive as long as they need it.  It’s for when you really want your traffic to make it from one system to another.  E-mail  uses tcp, because as I said before, people really want their email.