Zero Configuration: An Example

The potential for zero-configuration IP networking is tremendous. Consider the everyday task of printing. Once a printer is configured on your computer, it’s simply a matter of choosing an application’s Print command.

Take your laptop to a client’s company, or a neighbor’s house, and try to print something. If they have a printer that supports Bonjour protocols, printing is just as easy as it was on your local network. To print, connect an Ethernet cable from your laptop to your client’s LAN and start up your laptop. Or start up your laptop and it instantly finds your neighbor’s home wireless network. Either way, your laptop automatically discovers any available printers. You open the document, choose the Print command, and every available printer appears in the Print dialog. You select a printer, click Print, and the document prints.

Or say you want to play a network game with a friend. You open the game, and your friend’s copy of the game instantly sees your copy over the network. Or if you have a music sharing application on both computers, the programs themselves can discover each other and instantly swap songlists. Similarly, if you have a shared folder or have personal Web sharing turned on, your shared files and Web pages are instantly available to others.

This scenario is illustrated in Figure 1-1. In step 1, you open up your laptop in your neighbor’s house, and the laptop either obtains an address from the DHCP server in the router or, in the absence of a DHCP server, assigns itself an available local address. In step 2, the network is queried for available printers so that when you open the Print dialog, your neighbor’s printer is listed. Finally, in step 3, you turn on music sharing on your computer, and your neighbor’s computer sees it and connects.

These are just a few of the existing applications that can benefit from zero-configuration IP networking. Zero-configuration IP networking has the potential to enhance contact management, PDA synchronization, distributed processing, and many other network applications. Additionally, zero-configuration IP networking opens the door for a whole new class of IP-enabled digital devices.

Addressing

The addressing problem is solved by self-assigned link-local addressing. Link-local addressing uses a range of addresses reserved for the local network, typically a small LAN or a single LAN segment.

Self-assigned addressing is simply picking a random IP address in the link-local range and testing it. If the address in not use, it is now your local address. If it is already in use, pick another address and try again.

Note: Two hosts are considered to be on the same local link if, when one host sends packets to the other, the entire link-layer payload (the content of the packet as represented in the physical network, such as Ethernet) arrives unmodified. In practice, on an Ethernet network, this means that no IP router touches the packet between the two hosts.

Self-assigned link-local addressing has already shipped on IPv4 starting with Mac OS 8.5, Windows 98, and Mac OS X version 10.0. The IPv6 specification includes self-assigned link-local addressing.

Any user or service on a computer that supports self-assigned link-local addressing benefits from this feature automatically. When your host computer encounters a local network, it finds an unused local address and adopts it. No action on your part is required.

Hardware manufacturers should implement self-assigned link-local addressing on their devices to obtain the full benefit of Bonjour.

Naming

The proposed solution for name-to-address translation on a local network uses multicast DNS (mDNS), in which DNS-format queries are sent over the local network using IP multicast. Because these DNS queries are sent to a multicast address, no single DNS server with global knowledge is required to answer the queries. Each service or device can provide its own DNS capability—when it sees a query for its own name, it provides a DNS response with its own address.

Bonjour goes a bit further. It includes a responder that handles mDNS queries for any network service on the host computer. This relieves your application of the need to interpret and respond to mDNS messages. Just register your service with the Bonjour mDNS responder, and any queries for your name will be directed to your address automatically.

For name-to-address translation to work properly, you need a unique name on the local network. Unlike conventional DNS host names, the local name only has significance on the local network or LAN segment. You can self-assign a local name the same way you self-assign a local address—choose one; if it’s not already in use, it’s yours:

• Hardware manufacturers determine whether their chosen name is already use by sending an mDNS query for that name and looking for any response. If there is a response, choose another name. Devices without a user interface append an incrementally larger number to a default name until the name is unique. For example, if a printer with the default name XYZ-LaserPrinter attaches to a local network with two other identical printers already installed, it tests for XYZ-LaserPrinter, then XYZ-LaserPrinter2, then XYZ-LaserPrinter3, which is unused and becomes its name.

• Software services provide a local name when they register with Bonjour. If the provided name is already in use, Bonjour returns an error, and the service chooses another name.

Starting with Mac OS X version 10.2, users can set a local host name for their computers—the Local Hostname setting in the Sharing pane of System Preferences. The host name can be used anywhere a conventional DNS host name is used—Web browsers, command line tools, and so on. To indicate to the system that the name is a local host name, append a dot (.) and local. to the host name — Steve.local. is an example of a local host name.

For example, if a user types steve.local. into a Web browser, this tells the system to multicast the request for steve on the local network instead of sending it to the conventional DNS server. If a Bonjour-enabled computer named steve is on the local network, the user’s browser is sent the correct IP address for it. This allows users to access local hosts and services without a conventional DNS server.

For more information, see Bonjour and Domain Names.

Service Discovery

The final element of Bonjour is service discovery. Service discovery allows applications to find all available instances of a particular type of service and to maintain a list of named services. The application can then resolve a named instance of a service to an IP address and port number, as described in Naming.

The list of named services provides a layer of indirection between a service and its current IP address. Indirection enables applications to keep a persistent list of available services and resolve an actual network address just prior to actually using a service. The list allows services to be relocated dynamically without generating a lot of network traffic announcing the change.

Service discovery in Bonjour is accomplished by “browsing.” An mDNS query is sent out for a given service type and domain, and any matching servers reply with their names. The result is a list of available services to choose from.

This is very different from the traditional device-centric idea of network services. For someone who deals with servers, network devices, and network programming, it is easy to get in the habit of thinking about services in terms of physical hardware. In this device-centric view, the network consists of a number of devices or hosts, each with a set of services. For example, the network might consist of a server machine and several client machines. In a device-centric browsing scheme, a client queries the server for what services it is running, gets back a list (FTP, HTTP, and so on), and decides which service to use. The interface reflects the way the physical system is organized. But this is not necessarily what the user logically wants or needs.

Users typically want to accomplish a certain task, not query a list of devices to find out what services are running. It makes far more sense for a client to ask a single question: “What print services are available?” than to query each available device with the question, “What services are you running?” and sift through the results looking for printers. The device-centric approach is not only time-consuming, it generates a tremendous amount of network traffic, most of it useless. The service-centric approach sends a single query, generating only relevant replies.

Additionally, services are not tied to specific IP addresses or even host names. For example, a website may be hosted by multiple servers with different addresses. Within an organization, network administrators may need to move a service from one server to another to help balance the load. If clients store the host name (as in most cases they now do), they will not be able to connect if the service moves to a different host.

Bonjour takes the service-oriented view. Queries are made according to the type of service needed, not the hosts providing them. Applications store service names, not addresses, so if the IP address, port number, or even host name has changed, the application can still connect. By concentrating on services rather than devices, the user’s browsing experience is made more useful and trouble-free.

Avoiding “Chattiness”

Server-free addressing, naming, and service discovery have the potential to create a significant amount of excess network traffic, but Bonjour takes a number of steps to reduce this traffic to a minimum. This allows Bonjour to attain AppleTalk’s ease of use while avoiding any unnecessary “chattiness.”

Bonjour makes use of several mechanisms for reducing zero-configuration overhead, including caching, suppression of duplicate responses, exponential back-off, and service announcement, as described in the following sections.

Domain Names and DNS

DNS uses a specific-to-general naming scheme for domain names. The most general domain is . (“dot”), called the root domain, which is akin to the root directory / in a UNIX file system. Every other domain falls in a hierarchy below the root domain. For example, the name www.apple.com. is within the second-level domain apple.com., which is within the top-level domain com., which in turn is part of . (“dot”), the root domain. Figure 1-2 shows an abridged version of this hierarchy.

At the top of the inverted tree is the root domain. Below it are some of the top-level domains: com., edu., and org., and the local Bonjour “domain” local., discussed further in Bonjour and the Local Link. Below the top level are a few second-level domains, apple, darwin, and zeroconf. The tree can extend infinitely downward with, for example, www, at the third level.

You may have noticed that the trailing dot is left off of most domain names. The trailing dot does, however, have meaning. A domain name ending in a trailing period, such as www.apple.com., is known as a fully qualified domain name, much like an absolute path (such as /usr/bin) in a UNIX file system.

If you type wibble.apple.com into your Web browser (without the trailing dot), the system will treat it as an unqualified (partial) name and append the names from your list of search domains, such as apple.com., earthlink.net., myschool.edu., etc. The system will first try to append . (“dot,” the root domain), but if the name wibble.apple.com. doesn’t exist, it will continue down the list and try wibble.apple.com.apple.com., wibble.apple.com.earthlink.net., wibble.apple.com.myschool.edu., and so on, which is almost certainly not what you wanted.

Bonjour and the Local Link

Bonjour protocols deal in large part with the part of the network called the local link. A host’s local link, or link-local network, includes itself and all other hosts that can exchange packets without IP header data being modified. In practice, this includes all hosts not separated by a router.

On Bonjour systems, local. is used to indicate a name that should be looked up using an IP multicast query on the local IP network.

Note that local. is not really a domain. You can think of local. as a pseudo-domain. It differs from conventional DNS domains in a fundamental way: names within other domains are globally unique; link-local domain names are not. There is only one logical DNS entry in the world named www.apple.com., and because of the way DNS works, there can be only one. Host names ending in local., on the other hand, are managed by a collection of multicast DNS responders on the local network, so the naming scope is just that: local. There can easily be two hosts named meow.local. in the world, or even the same building, just not on the same local network.

Globally unique names are important and useful—in fact, they are one of the significant achievements of the Internet—but they require a certain level of administrative effort to set up and maintain. Local names are useful only on the local network, but in cases where that is adequate, they provide a way to refer to network devices using names instead of IP numbers, and of course they require less effort and expense to coordinate than globally unique names.

Globally unique names are particularly useful on local networks that have no connection to the global Internet, either by design or because of interruption, and on small, temporary networks, such as a pair of computers linked by a crossover cable, or a few people playing network games using laptops on the wireless network of a home or cafe.

If a name collision on the local network occurs, a Bonjour host finds a new name automatically (in the case of a device without a screen) or by asking the user (in the case of a personal computer).

Bonjour Names for Existing Service Types

Bonjour service types are named according to the existing Internet standard for IP services. The Internet Assigned Numbers Authority (IANA) keeps a registry of TCP and UDP protocol names and ports assigned to each, and Bonjour services are named according to this list. The list used by Mac OS X can be found in /etc/services, and the most current version of the list is on the IANA website at http://www.iana.org/assignments/port-numbers.

In order to distinguish services from domain names in DNS resource records, components of service type names include underscore prefixes. The format for Bonjour service type names is thus

_ApplicationProtocolName._TransportProtocolName.

The application protocol name is the official IANA-registered name for the protocol, for example, ftp, http or printer, as described above. The transport protocol name is either tcp or udp, depending on the transport protocol the service uses. An FTP service running over TCP would have a service type of _ftp._tcp. Services of this type would register DNS PTR records named _ftp._tcp.local. with their hosts’ multicast DNS responders.

Bonjour Names for New Service Types

If you are designing a new protocol to advertise as a Bonjour network service, you should register it with IANA at http://www.iana.org or http://www.dns-sd.org/ServiceNames.html.

The IANA currently requires that every registered service be associated with a “well-known port” or range of well-known ports. For example, http is assigned port 80, so that whenever you visit a website in your web browser, the application assumes that the HTTP service is running on port 80 unless you specify otherwise. This way, the port number for a website need only be memorized if the website is configured in a non-standard way.

With Bonjour, however, you don’t have to know about port numbers. Because client applications can discover your service with a simple query for the service type, well-known ports are unnecessary. At some point in the future, IANA may begin registering protocol names without requiring an associated well-known port.

Bonjour Names for Service Instances

Service instance names are intended to be human-readable strings. As such, you should name them descriptively, and let the user override whatever default name you provide. Because they are intended to be browsed rather than typed, service instance names can be any Unicode string encoded with UTF-8, up to 63 octets (bytes) in length.

For example, an application for sharing music over the network might use the local user’s name for a music sharing service, such as Ed's Music Library, by default. The user could override the default and name the service Zealous Lizard's Tune Studio, and the application would register a DNS SRV record named Zealous Lizard's Tune Studio._music._tcp.local., assuming the application’s music sharing protocol was associated with the name music.

Figure 1-3 illustrates the organization of the name of a Bonjour service instance. At the top level of the tree is the domain, such as local. for the local network. Below the domain is the service type, which consists of the protocol name preceded by an underscore (_music) and the transport protocol, also preceded by an underscore (_tcp). At the bottom of the tree is the human-readable service instance name, such as Zealous Lizard’s Tune Studio. The complete name is a path along the tree from bottom to top, with each component separated by a dot.