Preface

Two laptop computers sit less than two feet away from each other. They are so close they are nearly touchingand yet, until recently, as far as network communication is concerned, they may as well have been a thousand miles apart. Surely, communicating with a computer in the same room shouldn't have to be as hard as communicating with one on the other side of the planet? Our modern l6aptop computers bristle with an astounding array of communications technologiesEthernet, 802.11 wireless, FireWire, USB, Infrared, Bluetooth, and so onyet to move a file between two computers two feet apart, 99% of computer users still use physical media. They copy the file onto a floppy disk, burn the file onto a CD, or copy the file onto a USB flash-memory drive. For the 1% who do manage to move the file using networking, most do so by emailing it from one computer to another, which sometimes entails the file traveling to another continent and back, just to move two feet. To do that, the file has to traverse the slow connection to the global Internet and back, at a speed typically a thousand times slower than local Ethernet. Furthermore, a vast infrastructure of servicesDHCP, DNS, IP routers, SMTP relays, email servers, etc.all have to be working perfectly for the transfer via email to succeed. If the DSL line is down, why should that stop two computers sitting next to each other from communicating?

For computer novices, the situation is puzzling and frustrating. If you can see both computers, why can't the computers see each other? There are many ways of physically connecting two devices, but each way often requires its own custom software to do anything useful. If you have two computers with FireWire (also known as IEEE 1394), you can connect them with a FireWire cable, but...do you have any software for transferring files via FireWire? Do you know how to use it? You could connect two computers using the right kind of USB cable, but...would that do anything useful? You could aim the computer's infrared windows at each other, but...do you have any software for transferring files via infrared?

When we communicate across the planet, we use TCP/IP, and we don't care whether the physical connection is Ethernet or 802.11, DSL or cable modem, or a combination of those and other technologies. Could that give us the inspiration for solving the local communication problem? If we use TCP/IP, then it doesn't matter what the physical connection is, as long as it can carry IP packets.

The missing link here is that while TCP/IP is certainly powerful enough to solve the local communication problem, historically it was not easy enough to use, leading to the proliferation of different physical interfaces mentioned above. It was almost as if computer designers thought that if only we had enough different kinds of hardware, the problem would be solved. What we really needed were not more different kinds of hardware, but better software.

Zero Configuration Networking, or, as Apple calls it, "Bonjour," is that better software.

Zeroconf is the little missing link that makes TCP/IP on the local network as easy as USB. When you plug a Zeroconf camera into your Ethernet hub, it just shows up, as if by magic, in your Zeroconf web browser. It doesn't matter whether you have a working DHCP server. It doesn't matter whether you have a working DNS server. It doesn't matter whether you have a working connection to the Internet. Zeroconf works anyway, even when some or all of that infrastructure is not working. One of the benefits of FireWire and USB is that they can supply power over the cable, but with the advent of the IEEE 802.3af ("Power over Ethernet") standard, modern Ethernet hubs can do that too. A Zeroconf IP camera is as easy to use as a USB camera, except that because it uses IP, it's not tied to one particular physical connection technology. You can be sitting on the sofa with your laptop computer using 802.11 wireless, and the same Zeroconf Ethernet camera shows up in the same web browser, because even though they use different physical technologies, they speak the common language of IP. The web browser doesn't care what physical link-layer connection your computer is using. As long as it carries IP packets, it works.

Finding a TCP/IP printer on the local network is now as easy as using a directly attached USB printer. On Mac OS X, you just look in the print dialog and see a list of available network printers. Using Apple's "Bonjour for Windows," you just run the Printer Setup Wizard, and it shows you the same list of available network printers. If you visit a friend's house, one of your company's other office locations, or a hotel's business center, you don't need to ask for help finding the printer anymore. Just look in the print dialog, and there it is.

Returning to our file-transfer example, now that we know we have a working IP network, no matter what, we have a wealth of IP-based choices. Even the venerable old 1970's File Transfer Protocol (FTP) can benefit from Zeroconf. On Mac OS X, the built-in FTP server already advertises its presence using Zeroconf, and every third-party Mac OS X FTP client now uses Zeroconf to browse for FTP servers, so connecting to an FTP server is now as easy as running your preferred FTP client and picking the desired server from the Zeroconf list.

Other kinds of data sharing are easily facilitated using Zeroconf. If you want to let other machines and devices in your house play music from your computer, iTunes can advertise your music collection using Zeroconf. iPhoto can advertise selected photo albums using Zeroconf so that LCD picture frames hanging on your walls can display them. iChat can advertise your presence on the local network using Zeroconf, andwhile we're talking about file transferdropping a file onto a Bonjour iChat window is a more direct way of getting a file to someone than emailing it via the public Internet. Finally, if you're collaborating on a document with someone, a multiuser document editor such as SubEthaEdit (which allows multiple people to simultaneously edit a file) is a much more direct way to collaborate than transferring the file back and forth all the time.

All of these data-sharing applications were, in principle, possible before Zeroconf. However, the difficulty of making them work meant that, in practice, people were going to resort to the good old floppy disk. Furthermore, software like SubEthaEdit simply didn't get written back then, even though, in theory, there's no reason it couldn't have been. Local TCP/IP networking was like a complicated machine with no oil. All the right pieces were there, and it looked like it should work, but it took so much effort to get the wheels to turn that almost no one bothered. For communication on the worldwide Internetfor the Web and for emailthe pain of TCP/IP was clearly worth it, but for local-area communication people gravitated toward other solutions that offered the promise of better ease of use.

Zeroconf is not some huge, complicated piece of software. It's a small collection of simple ideas that act like the missing lubricant for the TCP/IP machine. Now that people see how easily the wheels turn, they suddenly begin to see all the useful applications of TCP/IP for short-distance communication around the home, around the office, and even around the desktop. TCP/IP is not just for the Internet any more.

Having transformed local-area TCP/IP networking, people realized that Zeroconf's DNS-based Service Discovery mechanisms could be taken back and applied to the wide-area Internet too. Being able to simply browse to find the list of printers on the local network was so useful that people wondered why they couldn't browse to find printers at a specified remote location too, and Wide-Area DNS Service Discovery was born. When staying in a hotel, Zeroconf allows you to see printers being offered by that hotel on its local network, and, with Wide-Area DNS Service Discovery, you can also see printers at your home or office, should you wish to print on one of those instead.

This book provides an in-depth look at the components of Zeroconf technology and a survey of the programming APIs that will allow you to Zeroconf-enable your product.