Internet communication an overview

INTERNET COMMUNICATIONS: AN OVERVIEW Suppose that you wanted to send a message to the authors of this book, but you didn’t have the postal address, and you didn’t have any way to look up our phone number (because in this example you don’t have the Internet). You remember that we’re from the UK, and London is the biggest city in the UK. So you send a postcard to your cousin Bob, who lives there. Your cousin sees that the postcard is for some crazy hardware and technol- ogy people. So he puts the postcard in an envelope and drops it off at the London Hackspace because the guys there probably know what to do with it. At the Hackspace, Jonty picks up the envelope and sees that it’s for some people in Liverpool. Like all good Londoners, Jonty never goes anywhere to the north of Watford, but he remembers that Manchester is in the north too. So he calls up the Manchester Digital Laboratory (MadLab), opens the envelope to read the contents, and says, “Hey, I’ve got this message for Adrian and Hakim in Liverpool. Can you pass it on?” The guys at MadLab ask whether anyone knows who we are, and it turns out that Hwa Young does. So the next time she comes to Liverpool, she delivers the postcard to us. IP The preceding scenario describes how the Internet Protocol (IP) works. Data is sent from one machine to another in a packet, with a destination address and a source address in a standardised format (a “protocol”). Just like the original sender of the message in the example, the sending machine doesn’t always know the best route to the destination in advance. Most of the time, the packets of data have to go through a number of intermediary machines, called routers, to reach their destination. The underlying networks aren’t always the same: just as we used the phone, the postal service, and delivery by hand, so data packets can be sent over wired or wireless networks, through the phone system, or over satellite links. In our example, a postcard was placed in an envelope before getting passed onwards. This happens with Internet packets, too. So, an IP packet is a block of data along with the same kind of information you would write on a physical envelope: the name and address of the server, and so on. But if an IP packet ever gets transmitted across your local wired network via an Ethernet cable—the cable that connects your home broadband router or your office local area network (LAN) to a desktop PC—then the whole packet will get bundled up into another type of envelope, an Ethernet Frame, which adds additional information about how to complete the last few steps of its journey to your computer. Of course, it’s possible that your cousin Bob didn’t know about the London Hackspace, and then maybe the message would have got stuck with him. You would have had no way to know whether it got there. This is how IP works. There is no guarantee, and you can send only what will fit in a single packet. TCP What if you wanted to send longer messages than fit on a postcard? Or wanted to make sure your messages got through? What if everyone agreed that postcards written in green ink meant that we cared about whether they arrived. And that we would always number them, so if we wanted to send longer messages, we could. The person at the other end would be able to put the messages in order, even if they got delivered in the wrong order (maybe you were writing your letter over a number of days, and the day you passed the fifth one on to cousin Bob, he happened to visit Liverpool and passed on that postcard without relaying through London Hackspace or MadLab). We would send back postcard notifications that just told you which postcards we had received, so you could resend any that went missing. That is basically how the Transmission Control Protocol (TCP) works. The simplest transport protocol on the Internet, TCP is built on top of the basic IP protocol and adds sequence numbers, acknowledgements, and retrans- mis

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