Subject: how the 'net works, part II Fri May 08 19:01:48 1998 > I figured my presentation would go along the lines of: > > 1. brief history of the net. The US military requirements, etc. there are several good histories of the 'net online, but frankly i'd suggest you look at the first few pages of RFC791, which defines the Internet Protocol: ftp://ftp.isi.edu/in-notes/rfc791.txt and RFC793, which defines the Transfer Control Protocol: ftp://ftp.isi.edu/in-notes/rfc793.txt the technical stuff gets pretty opaque, but they both have extremely good introductions which lay out the purposes and logic behind the whole 'net. both documents were written for approval by the DOD, which means that at least part of them had to be accessible to an audience that didn't have the faintest idea what the techspeak was all about. as an aside, both RFCs predate the OSI model, and use a simpler network model of their own. you may prefer to show that to your people, because it frankly glosses over several of the more fiddly details which were built into the (later) OSI model. > 2. Packet switching and why this met the needs of the military. again, the RFCs will give you a good idea of how the military looked at it. to understand packet switching, it helps to start by defining the term 'switching', then go on to compare it with the other way of handling communication, namely circuit switching. the best way to define circuit switching is to go back to the old-style telephone systems which gave it that name. if you think back to your favorite old movie that has a telephone switchboard operator as a character, you have an example of circuit switching in action. the office building has, say, 10 telephone lines coming in, and there are 100 telephones on various desks around the building. for every phone line comain in, there's a plug on a retractable cord. for every phone, there's a socket in the plugboard. the operator sits at the plugboard listening for incoming calls. when someone dials in, a bulb lights up above that plug. the operator connects the plug to her own headset long enough to find out who the caller is trying to reach, then she plugs it into the appropriate socket. as long as that plug stays in that socket, the two phones at either end are connected to each other. nobody else can use either the plug /or/ the socket as long as the two are hooked together. on the positive side, any of the 10 lines coming in can connect to any one of the 100 phones in the building. on the negative side, there can only be (at most) 10 incoming calls at any time. the ability to connect any plug to any socket is the 'switching' part. human operators were eventually replaced by automatic relay matrices called 'switches', which is where we got the name. the fact that you have to have an unbroken electrical circuit between the phones at the two endpoints is where we got the term 'circuit'. the point of a circuit switching network is that it has a predefined number of channels for communication, and operates by connecting and disconnecting endpoint systems from each other. it's an all or nothing system, because you're either connected or you're not. packet switching is more like the mail room in the same building, but the metaphor isn't quite as obvious. you have roughly the same setup as with the switchboard as far as input sources and destinations.. there are a set number of mail deliveries during the day, and the walls are lined with pigeonholes for each person in the building. you also have a person who takes the input (an incoming letter) and routes it to the correct output (a pigeonhole). (aside: has everybody seen _The Hudsucker Proxy_? the old guy in the mailroom, throwing envelopes into pigeonholes from 10 feet away at a rate of about 2 per second, is simply amazing) now.. assume that the mail is coming from several different sources. one set of bags will be from the US post office, another will be fresh off the boat from overseas, a third will be from a private courier, and the fourth is internal correspondence from those pneumatic-tube thingies. each source is roughly equivalent to an incoming telephone line. you only have a certain number of channels, but this time each channel can carry several different messages simultaneously. the handsome executive on the fourth floor (who's toying with the affections of the switchboard operator, she's far too good for him, and will eventually end up with the keen-eyed young chap in the mailroom, as soon as her head's no longer being turned by the exec's smooth city ways) can be corresponding with a dozen people through each channel (sucking up to the execs above him, stealing the work of the engineer on six, trysting with his femme fatale in Paris, and arranging a deal to have the apartment building where the operator's widowed granny lives torn down.. the usual stuff) without having a monopoly on any of the channels themselves. the thing which makes it all work is the fact that every letter comes in an addressed envelope. the envelope is a packet. it binds a burst of information together, and tells the router (the young mailroom hero) how to deliver it without having to see any of the content. once all the envelopes have been addressed properly, you can dump them in a bag and let the guys in the mailroom sort them out. there's no upper limit to the number of simultaneous connections, like circuit switching has, but the performance degrades as the load increases. (the movie resolves itself after the mailroom chap 'accidentally' mixes up a couple of letters. the president of the company learns about the apartment being torn down.. and that the operator's grandmother is an old flame from back in 'ought-four. the femme fatale discovers the engineer, realizes he's a genius, and her passion for particle physics, with which she had become disillusioned, is reawakened. she calls to break off with the exec, and unwittingly shatters the operator's dream-bubble. the operator dives headlong into the arms of the mailroom chap, and that sneaky Machiavellian bastard has the good sense to keep his mouth shut and just look noble as the (former) exec is escorted out of the building.. blackout.. roll credits) the internet uses a packet-switched delivery system, and provides reliability through redundancy. when one computer sends a message to another, it keeps sending copies of the same packet until it gets a confirmation from the other end saying, in effect, "okay, i got it.. you can shut up now." and i'll get to the rest in another couple of days.